It happened only with children and teens so far. When a sim does something that increases a skill, sometimes the bar continues to increase even after the sim stopped doing it.
Usually it happened when a child sim learns two skills at once, ie video games + one of those children-only skills. But a few days ago a teen sim played chess with a young adult sim, and after the adult won (having genius trait and 10 logic, duh), teen's logic bar continued to increase even when they stopped playing.
It's a strictly cosmetic issue, the skill does not increase with the bar. The bar remains until you reload, go to another location or the sim goes to school.
* I wish I could delete my posts here, just noticed that I posted it in the wrong thread by accident, sorry 🙂