So I did a little more digging, and found that this actually first cropped up in Ubuntu 17.10.
I then ran mozregression (back on 18.04, my main desktop) and here’s what it found:
7:12.48 INFO: Last good revision: 64bab5cbb9b63808d04babfbcfba3175fd99f69d (2017-10-25) 7:12.48 INFO: First bad revision: aa958b29c149a67fce772f8473e9586e71fbdb46 (2017-10-26) 7:12.48 INFO: Pushlog: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=64bab5cbb9b63808d04babfbcfba3175fd99f69d&tochange=aa958b29c149a67fce772f8473e9586e71fbdb46
After that ”There are no build artifacts on inbound for these changesets (they are probably too old).”
As this was my first time ever using mozregression, I have no idea how useful that was, but if there’s some way I can narrow this down further, I’d be happy to.