Sorry to revive a possibly dead thread, I just wanted to add my two cents to the "Official" recap/answer to this problem. I've been having this issue as well, I'm running 3ds max 2017 (Don't really know what service pack and never been able to remember how to read it, my bad, but this is what I get from the maxVersion() command at least: #(19000, 48, 0)). Its been an issue bothering me for a week, but I think I found a solution to it!
In the edit poly modifier (if multiple edit poly's probably everyone) make sure to check "Preserve UV's" and make sure that the right channel is selected etc. Then after applying the unwrap modifier and seeing it break, hit "Reset UVWs" in the channel dropdown. It then seems to snap back to the unwrap that was present before.
I really don't wanna update to 2018 and later, love the big 3ds max button and quick access to recent projects too much XD
Hopefully, it's a working solution that can benefit someone having this issue ๐
Edit: It does also seem to work even if you don't have the "Preserve UV" checked in the edit poly modifier.