KiCad-nightly V5.99 is indeed still not stable, but addition of new features has stopped some time ago, and a lot of effort is now going towards making it stable for an official release. So there are bugs, but they likely are small and if an annoying bug is discovered it will very likely get fixed quick (days or weeks). The record for a bug I reported was 34 minutes from report on gitlab to a committed fix.
I was surprised that KiCad V5.1.10 was released, as it recall to have read that V5.1.8 would be the last of the V5.
My previous posts were about figuring out what was happening and that is pretty clear now. If this bug was still reproducible in KiCad-nightly V5.99 I already would have filed a bug report for it, But for V5 and this close to V6 it does not seem worthwhile.
You already have one workaround, by disabling the thermal reliefs. Another simple workaround is of course to just draw a track from the pad.