MAX(values) doesn’t work properly in the scenario where you rollup a date field which uses formatting. It’s no longer recognised as a date and can cause problems.
It also looks like old field level formatting is stored when a datatype changes. I used an existing field that was formatted for currency and changed the formula to simply show a date field. My rollup in the parent record caused a major problem later on although the child field appeared as it should: I was locked out the parent table as it got stuck on the ‘Loading’ screen.
To re-enable access to that table I had to change the formula in the child table.