Did your ex propose it, or did your fiancé start it? Is that clear? Because the source of that “sudden development” reveals intentions directly. If your fiancé stayed passive, it points to a gray area like “I’m not uncomfortable, but I also don’t want to get involved.” This situation is a bit more complicated; he could have shown a clearer stance.
Did you say that you question the impact of what you call “spontaneous” on your feelings? Because sometimes people completely overlook what their actions mean to the other person. Especially in a relationship, it’s important to discuss how such actions are felt in terms of relationship dynamics; otherwise, spontaneity remains an excuse, hanging in the air.
So, how was the general state of the room? The atmosphere’s tension, the distance with the ex, general postures, etc. I mean, before the spontaneity of the dance, did these already convey a sense of ease? We also need to talk about the dynamics as much as the spontaneity.
Aside from the incident of dancing with the ex, have there been past examples of your fiancé acting “without thinking” like this? Because if this is the first time, it could be testing the response. But if it’s a habit, then there might be a more deep-rooted issue related to boundaries.
Let me ask you this: Did he see you while dancing with his ex? Because if he continued that behavior after realizing your presence, it would basically be a way to test you or ignore your reaction. If he didn’t see you, it might be a bit more innocent distraction, but if he did see and continued, that would really bother me.
Was there something else bothering the ex while dancing? I mean, is it just the comfort of the fiancé involved, or is this dance something strange for both parties, like an energy from a past connection? The other party’s attitude could also provide some clues.
What was the reaction of others while dancing with their ex? Did anyone nearby talk about it at that moment, or did everyone act as if nothing was strange? Because sometimes, even an external reaction can bring up things that people might not notice. If someone else commented in that moment, what did they say? Please evaluate this as well.
So how did your fiancé express himself when he was telling you about this dance incident? Was it guilt, defensiveness, comfort… What tone did he use? Because sometimes, more than behavior, the way someone recounts an incident can reveal how they perceive their own actions.
Did Sana use a defense like “but there’s nothing” when talking about it? Because a sentence like this usually hides an attempt to downplay the situation. Even if it doesn’t seem important to them, it needs to matter to you, and they should understand that. If they brushed it off with something like “oh well,” then the way they communicate with you is a bigger problem than the dance itself.
Could a photo or video have been taken during the moment? Because sometimes people don’t realize how their behavior appears to outsiders, but if such a thing exists and you watch it, your partner’s facial expression and demeanor at that moment could give you a clearer idea. Maybe you could watch it together and get their opinion as well.
What was the relationship of your fiancée with their social circle like? That is, were there acquaintances on their side that were more dominant in the environment? Because if they were in an environment where they felt alone, dancing with an ex might have developed as an unconscious relaxation or search for belonging.
It’s been talked about how the ex-girlfriend approaches this dance, but what about your fiancé’s approach to the ex? For example, does your fiancé see this dance event as “friendly,” or is he dragging some past energy into it? Because this is completely related to his intention. There’s a huge difference between someone who considers being in the same environment as an ex to be normal and someone who mixes in the past.
And here’s an interesting question; does your fiancé have any kind of connection with this ex at the moment? I mean, is there still social media following or any shared activities in between? Because this dance might also be coming from a place where the past hasn’t completely closed. The old energy you mentioned sometimes doesn’t just appear in the moment.
How was the timing of your fiancé’s explanation about this dance situation? Did they tell you right away that day, or did it come up when you asked? If they shared it spontaneously, it might have been something ordinary for them, but if they explained it after you noticed, they might be a bit inclined to hide things. This is also an important clue.
Why are you still in the same environment as your ex, for example? Does this happen often? I mean, it seems strange to me that two people with a relationship history can be so casually close.
Is this ex-lover situation so “normal” for your fiancé, or did others in the room react the same way when they saw the dance? I mean, is the issue his own comfort level, or does he generally receive social approval for such things? Because sometimes a person shapes their own boundaries based on the silent reactions of others.
So, was the decision to dance with an ex a spur-of-the-moment suggestion, or was there planning or a proposal beforehand? Because in spontaneous situations, it’s possible to act without thinking, but if it was discussed in advance, you might question your fiancée’s intentions or comfort boundaries. It makes a difference in terms of the dynamics of both.
What was your fiancé’s attitude towards this dance situation after you reacted? Did they go on the defensive, or did they try to understand directly? Because their approach to the relationship after your reaction reveals their overall stance better 
Did he really not foresee at all why you might be uncomfortable with something so “smooth” for your fiancé? I mean, isn’t normalizing a past relationship so easily a bit contradictory to respecting the current one? Is it like saying, “This is how I am, deal with it”? 
It’s normal to feel uncomfortable while your fiancée is dancing with her ex. She should have realized how this situation would affect you. Maybe there are other things as well; does she exhibit any other behaviors that make you feel this way?