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You're right, it's from the recent update, which improved some of the basegame traits. One of them was probably goofball trait, with which sims will autonomously prank objects like toilets.
Onto the part of stopping that - if you're using MCCC and have MC tuner module, then click on your sim, then go to mccc > mc tuner and enable autonomy scan, then click on the toilet and use the option to search all autonomous actions on this object. Locate the prank interaction (should be in these brackets [ ] and click on it, so its autonomy will be disabled.