I believe the first time it happened was when Fero turned into a wolf early on in season 3 - it may even have been as part of his love letter? I can’t remember for sure. Austin just mentioned in passing that Fero couldn’t detect any wolves nearby after the failure.
If I had to guess, I think you’re right that it must have been failures at the end of Nacre that lead to the problem, but I’d have to relisten to be sure (and, sadly, I don’t have the time to do that). I think Austin laid what was happening out a bit more clearly once Fero wiped out that particular type of sand towards the send of season 3, but I don’t think he laid it out step by step. Hopefully someone else here will have a better memory/understanding than I do!