I'm raising this as a bug and I think there are potentially two issues here:
We have a member who posted a message to our Committee subgroup and for some reason, also copied the email to an address that gets forwarded to the same group via an integration. The copy of the message sent direct to the group was sent to other members and the copy sent via the integration has been held pending approval (part of our moderation process). A recipient and member of the same group then replied to all and so now we have the two messages showing on the subgroup archive, and two messages pending approval.
Seeing the pending messages and not realising they has also been posted directly to the subgroup I tried to approve them. There is no warning message to say that approval has failed but the messages stay as pending. I could probably delete them but have left them for now as evidence of this issue.
In addition, the member who posted the original message received the following response from Groups.io:
This was very confusing for our member as it indicates that he is not a member of our Committee subgroup when he is.