we were at the ice caps getting blubber from seals, but then randomly when day broke all the ice instantly started melting at the same time

Are you sure this isn't just because you are holding a torch with some mod that increases light level for held items? I can't reproduce this, and it doesn't look like TFC sea ice melting behavior is any different from minecraft ice, except for the fact that it turns into saltwater

EDIT: Also worth noting that I tested this in a temperate climate where sea ice wouldn't naturally spawn to see if it could be something with the snow/ice system messing with it, and that did not appear to be the issue

ive narrowed it down to a mod issue, it doesnt occur with only tfc and patchouli installed. im going to run through them and see which mod it is so i can report it to them

it is cold sweat, my first guess and one of the few content mods i had installed