Yes, there is a known issue with Mastodon folks that have joined forte groups. This occurred during the conversion to conversation containers around a year ago. The fix would be to figure out what folks are using Mastodon and send them an Announce activity for group comments, since Mastodon doesn't support collection operations. This isn't terribly difficult but has not yet been done.  And there is no solution for Mastodon participation in private groups, which can't use the Announce method.

It would be better and easier to implement this if Mastodon supported groups and private groups, and we could just emulate their existing mechanisms, but they do not. They don't actually support groups and audience control and reply control. They do support boosts, so that's all we have to work with. You're welcome to file an issue, but being a volunteer project there is no telling when this might actually get implemented. At this point, nobody has been concerned enough to file an issue, so as a consequence, nobody is working on the problem and it hasn't even been properly documented. The lead developer is retired and contributes very infrequently these days. Looks to me like about 3-4 days of effort  to sort this out. Maybe a bit more. Private groups and circles/aspects/lists will remain unavailable to Mastodon folks, but this will bring back their ability to interact with public groups, which they could do from around 2018 to 2025 when we upgraded to conversation containers.