Hi Glenn Maxwell,
First of all,I would like to answer your question, users can have up to 1,000 groups, but there is no official documentation on exactly how many unified groups can be displayed in Outlook.
Then, I would like to verify that when this Unified Group was created , Because it takes a while for newly created groups to synchronize to Outlook. Please also check whether other group members (who have less than 15 Unified Groups in outlook) can see this Unified Group properly. If the group is visible to other members, then I don't think that's a reason to "hide the group from the global address list" and its unified groups.
I suggest you to remove the user from this group and re-add the user to see if the problem is solved, if the issue persists, please feel free to give me a comment, I'll be glad to help you solve the issue!