The happiness calculation in the Tavern slider does not account for corruption. I don't know if this affects the Museum too.
Test city has level 47 Tavern, level 21 Museum, and level 38 Town Hall with 7% corruption (level 13 Palace, level 12 GR). The system of government is Theocracy and Faith is 100% and conversion rate is over 100%.
To reproduce the issue build some units in a town that is suffering corruption - really, anything to reduce the population. Then whack the Tavern slider on full to recover the population quickly. Once the population has recovered go to the Tavern and reduce the slider so the smile turns to a thin line and click Cheers. Happiness is then recalculated downwards with corruption applied and in my test city the population becomes angry. Adjust the slider upwards and click Cheers again to resolve this.
This issue does not arise in cities without corruption.