I got a reply from Renato Rivero - they claim to have made a mistake. I'm not sure we know what really happened, but they said they won't contribute any more anyway, and I have already removed their access to the translation.
I compared the current state of the translation to the last update I used and the current state is further ahead. It looks like it preserved all new translations when I restored the old translation, and there is no significant loss. So I don't think anyone's work has been lost. Also thanks to nemezes, it looks like there are no more mistakes currently. So right now Portuguese is on 80% (3% further ahead than the translation I restored) and everything looks OK.
Unfortunately I can't find a way to recover the contribution counts; I'll contact POEditor's support and see if they can restore those. As long as they can tell us the count as it was on January 23rd, we will simply add those contributions to the results when it's completed.
Thanks for your patience everyone and sorry for the inconvenience. I think it's safe to keep contributing; after everything that happened it doesn't look like we lost any actual translations, so even if it happens again it looks like we should be able to restore everything. However it's definitely understandable if anyone wants to wait a couple of days before continuing to make sure it's settled down.