I think I’m going to try a git bisect, I’ve never actually used it anger before, so it’ll be a learning experience 
My USB stick didn’t work at first, then started working with the delay fix, and has now stopped. I’d like to get to the bottom of it. Looking at the code, I have a suspicion the issue may be a change in libavr32, but hopefully I’ll be able to nail down an exact commit with bisect.