Nothing, just a way for guests to express their dissatisfaction.

I have seen upside-down bowls on tables in some restaurants, which is a way for customers to vent their dissatisfaction. That restaurant has improved its service accordingly, and now its business is booming. I often go to eat. )

It should be pointed out that this guest is so incompetent that it is neither civilized nor hygienic to put the bowl upside down on the table when eating. As for taboo, it is probably a kind of rhetoric that people used to stop this kind of behavior before, so it is not a worry.