These problems could simply arise from the fact that Mastodon is starting to really work on the management of Activitypub groups.

And re-invent the wheel once more, preferably in a proprietary way that's as incompatible with the rest of the Fediverse as possible but sold to the Mastodon users as the Fediverse gold standard.

It is therefore appropriate to report any incompatibilities to the Mastodon staff, so that they can claim the other side is broken unless the other side has rock-solid proof that it's actually Mastodon that's doing it wrong.

FTFY