Kissing and makeout interactions stay queued up, but they won't act on them if they're in the hot tub. I haven't tested this with all hot tubs yet. I'm experiencing it with the new AA hot tub.
I've tried resetting the sims and having them move into different spots in the hot tub, but have experienced the same result: kissing stays queued up for multiple sim hours and has to be manually cancelled by the player. Their relationship is high enough that they have the interaction option to kiss or make out, but they won't. They will perform other interactions while in the hot tub, like flirting, telling jokes, and chatting.
I'm on PC. No mods or CC.
