It's definitely not custom content. I do have some mods but I didn't check if they are UI.
Are screen slam mods considered UI? because that was one of my most recent downloaded mods and has not been updated since 2019. It's simply to remove the screen slam. I also have a faster skills mod as well that was NOT the same creator. The screenslam creator made a faster skills mod, but I didn.t use that one. I removed that mod to keep safe.
And I don't have UI cheats extension either.
When I removed the mod and added a Child Sim to the game, it stopped appearing when the child(ren) was building skills. It worked on 2 kids so far.
