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This is a limitation in Mastodon, because it doesnt expect that there are two accounts with the same name on one instance. With Lemmy that is possible, because users and communities are in seperate namespaces. So in this case, Mastodon seems to prefer the user, even though the community is listed first for webfinger. You can try searching for the community url (https://lemmy.ml/c/openscience). If that doesnt help, you can open an issue.

Then it seems like a bug in Mastodon.

I think Mastodon gets confused by the fact there is a community /c/openscience and a user /u/openscience on lemmy.ml