On my completely new (PC) vanilla save with eight sims (basically clones of each other as its a test save with no mods) while they are all stuck in their rooms three of them ended up with an emotion which made no sense because I'd just started the save and put them in their rooms to begin testing out a different bug I was experiencing.
I clicked on the first one and he is flirty, the second sim was embarrassed and the third was confident, the cause was milestones apparently. I checked their milestones, they had none generated because they were created and plopped down by me less than thirty real life minutes ago, it's still Sunday in the game on the first day of this new save. They have no milestones! How are you getting emotional moodlets.
I've played mod heavy for years but this patch in August has made me go vanilla due to how badly it wrecked the mods. I didn't realise the random milestone moodlets were actually the game bugging out - I've been thinking it's mod related for a long time, as various sims would randomly get a moodlet that made no sense to the sim but I've ignored it due to it being just one moodlet amongst many mod moodlets that overpower the game's moodlets. It was shocking to me to realise this bug that's annoyed me is actually from the game itself.
How is it possible for newly generated YA sims to get milestone moodlets when they have no milestones? So in summary, out of my eight sims, within twenty sim minutes three of them ended up with random milestone moodlets which influenced their moods, while the others were of the emotion of "fine".