Sim going to wash dishes gets reset

Do you have any LastException files in Documents\Electronic Arts\The Sims 4 ?

- Delete the ones you have
- Trigger the error again
- Check if new ones are created
- If that is the case, please copy/paste the content here. You can open the file with any text editor.

Deleting the file won't fix your issue. Posting the info is meant for troubleshooting. 

Ok I've done what you asked but it didn't create any new LastException

It appears there was a glass of water they were trying to fetch (to wash with the rest of the dishes). Once I got rid of it there was no problem anymore. But the bug seems to be that if they go downstairs with dishes in their hands to go get other dishes on another floor, they get reset 

Hi Nawaahhh​ 

could be this is happening when they can't reach certain dishes. They used to just put the dishes somewhere if that was the case but maybe even that wasn't possible in your case.

Where was the glass placed ?

It was placed on toilets of the first floor. They can reach it. I'll test later if the bug happens again when placing another glass of water there, if it's useful