Lemmy groups automatically boost any new posts so you will see them appear in your Mastodon feed. Worst thing ever. Mastodon boosts any posts from the community in the local community-as-user, but it treats replies as posts too, making for one spammy feed. Instead of that, I’d suggest following the hashtag automatically attached to the community,

This isn't a bug, but a feature of microblogging systems like Mastodon... Today, however, Mastodon users can use an app like Raccoon that allows accounts to be represented as "group" accounts so that discussions are displayed as topics: https://poliversity.it/@macfranc/116736605177662762