@bristle Pro tip – when replying to a Lemmy thread from Mastodon, remember to tag the community (i.e. @tech) in the reply in order to ensure your reply actually shows up on Lemmy. If you don't, it won't always federate back to Lemmy.

It seems to federate without tagging when the thread OP is a native user on the Lemmy instance, and when directly replying to a Lemmy user. Still unsure exactly when it will and won't.