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 seems lemmy upvotes dont seem to federate at all with mastodon. Replies seems to federate but for some reason 1 reply from mastodon is missing on lemmy.