@MeggersNCat
When your sims cancel their tasks, what are they doing instead? Are they nothing or is the game adding something to the sim's queue? As @crinrict said, even with autonomy off, the game will push actions on a sim. Most do not interrupt the current action, but I have found some things still do. For instance, if you have set a like for any decor for a sim, the sim will get pushed to judge decor almost constantly. This push breaks a lot of things that normally don't get interrupted, like meditation or painting and a bunch of other things you might have a sim do. So, in that case, I took away all the decor likes for my sims and that stopped it. There may be other likes that do that too but Idk for sure.
Hope this helps.