Would _you_ leave in this case? :P
I mean, eventually, maybe.
Does the timing of setting it matter? When I just tried they would just leave like usual.
This only happens when you keep pressing on the bathroom need to be max, he will always stay at the bathroom, but if you only click on it once, he just leaves in 10 seconds when his bladder is empty :) So, this is not a bug and can be closed.
It's expected guests stay in the toilet if you keep refilling their bladders ;).