This is likely a bug
We have a closed group with ‘request’ membership.
Group is visible to everyone and mentionable by everyone.
When someone requests membership, group owners invite the whole group to the membership message for review and (whiper) discussion.
When the group is invited, the membership request message is notified of that invitation, and the individual removal of each group owner. That is fine. Since group owners are members of the group, we expect them to be removed individually from the notifications when the group is notified. OK.
When inviting the group, we expect that all members get access to that message, not just owners.
So once a group owner have invited the group, they send a message mentioning the group, telling them to evaluate the membership request.
At this point, when we mention the group, a popup warns us that
You mentioned [the group] but only NaN members will be notified because the other members are unable to see this personal message. You will need to invite them to this personal message.
It seems like the group membership requests are specifically restricted to group owners and break the normal topic invitation flow.