It's worth nothing that this machine is now running the amd64 arch of 5.7 on a replacement apu1d4, and I'm not seeing this issue anymore under similar conditions (configs were all 1:1 copied).
Who knows which change (hopefully) resolved the issue, but maybe this can serve as useful info in the future if anyone else has a similar issue. Sorry for not having more time to troubleshoot, but thanks for another awesome release :)
