AITA for the joke we played on my friend and the guy he was with?

19m good friends with this guy 18m name him Alex, we went out with other friends and went back to my building for afters. Alex has recently started speaking again with a friend of his from school name him peter. there was always like a recognised tension between them but they didn’t do anything about it and stopped talking when they went to uni but are now back chatting and met up a few times apparently. Alex brought Peter that night, they end up making out a bit and then he also came back with us to my flat.

They disappear and I hear from another friend idk how he knew this that they’re making out on the stairs. Specially the 11th floor. building has 11 floors there’s lifts that go into these connector rooms with doors to each flat and also there’s stairs that run up into each of these connector rooms too. But on my side of the building on the 11th there’s no flats just locked doors. So they were up on the stairs outside the 11th floor because well no one’s going to be up there at that time.

Earlier we were talking to my flatmate who’s ex Jehovah’s Witnesses and someone else jokingly asks if they do that thing those American Mormons do on TikTok where they jumpscare people doing more than holding hands in public. So hearing what Alex and Peter are doing i, as a joke, suggest we go do that to them.

So we all take the lift up to open the door to the stairs from that side to surprise them. Now I didn’t see personally I was behind them but apparently Peter was zipping himself back up and Alex was getting up from a lowered position. We all basically run away to the lift with the others laughing loudly but I wasn’t.

I speak to Alex the next day, he explains himself that his building is very strict with not signed in guests after 11 so they couldn’t go back there and it was peters idea. (My buildings really chill tbf he’s not wrong)

Then he goes on about how mad at me he is. I tried to explain it was a joke and it’s kinda on them for doing that on a stairway? But he seemed genuinely upset not just at the embarrassment but also he apparently really likes Peter and we ruined whatever was happening there, not in terms of how they’re getting on they’re still chatting and I think potentially getting a date sorted. Specifically Alex getting to suck him on the stairs getting ruined was what he was sad about, he didn’t put it like that but it’s what he meant.

I did say i agree it was an annoying thing to do even if they were just making out, but it’s not something he should be getting worked up other either. First of all when we left they could’ve continued, and I tried to lighten the mood by saying I’m sure Peter will be happy enough to give him the chance again. Also I probably did him a favour because Alex is a virgin like completely and if he really likes this guy, first time doing something maybe could be somewhere little more intimate? Alex gets even more mad at that and conversation ends.