Having thought about this for a month now, and seen both in action, I have an answer.
Any ‘bug’ requiring any kind of discussion whatsoever belongs on meta.discourse first.
If the bug is …
- totally straightforward
- purely technical and not sociological in nature
- no reasonable person would dispute its status as a bug
- unlikely to need any discussion, other than “there it is” and “here is how to fix it”
… then it can go on GitHub Issues.
Note that this is a very narrow and strict definition, so unless your bug strongly meets all 4 above criteria, please keep it here on meta for discussion.