It’s a bit confusing because each action on the Fediverse uses the same universal language, but each service translates that language into their own specific rolls for their service.

Lemmy upvotes use “Like” which is common language in ActivityPub but down votes use “Dislike” which is not* also used in the ActivityPub language. It’s up to the other services how they interpret that. Mastodon favourites use “Like” but it has nothing for “Dislike”. Mastodon boost uses “Announce” which is what Lemmy uses for posts and comments to a community.

There are also some actions that use the ActivityPub language on both services but one simply just doesn’t send that action to the other service. Mastodon users can follow Lemmy users but Lemmy users cannot follow any users. I think Mbin does allow following users.

From the Lemmy ActivityPub documentation

An upvote for a post or comment send an ActivityPub Like action

Users can send posts or comments to a community which the community forwards to its followers in the form of Announce.

From the Mastodon ActivityPub Documentation

ActivityPub Like is transformed into a Favourite.

ActivityPub Announce is transformed into a boost on a status

So this post has (at the time of writing this post):

• 0 favorites on mastodon
• 101 upvotes on lemmy (2 downvotes)
• 9 replies on mastodon (7 lemmy instances, 2 mastodon)
• 8 replies on lemmy (7 lemmy, 1 mastodon)

From this I would say it looks like lemmy upvotes dont federate at all with mastodon. Replies seem to federate but for some reason 1 reply from mastodon is missing on lemmy.

I’m more confused after reading the responses so far than I was before coming here.