In both upstairs and downstairs bathrooms, whenever we flush the
toilets, and then immediately go to the sink to wash our hands, the
water coming out of what is supposed to be the cold faucet is very hot.
After about 30 seconds, it will go back to cold.

Recently, we had a flood in our upstairs when the black floating ball
in the tank of our toilet broke while we were not home. 80 gallons
were deposited below. When repairing the toilet, the plumber noted
that there was hot water filling up our tank.

Does anyone have any ideas for these two things to be happening? I
don't know if they are related or not, but I am open to suggestions.
The plumber thought that the filter in the shower might need to be
changed and could be causing the issue.

Thanks in advance,

Collins