Aaaaarrrggghh, cursed behavior from XF1 and other systems where time limit 0 previously always meant disabled. I just spent X time being frustrated on why my regular test user on XF2.1 migration test site couldn't edit anything. You changed how this behaves.
If there is a permission that explicitly grants editing and the time for editing is zero, it shouldn't then contradict that explicit grant.
/beer induced rant off