There seems to be some issue with the Outlook client, as detailed in this thread: https://learn.microsoft.com/en-us/answers/questions/745550/unable-some-users-are-unable-to-open-34shared-cont.html?childToView=811636#comment-811636
Can't add shared contact to outlook client the user already has owner permission on contact folder

Hi, We have a shared mailbox and populated it's default contact folder with contacts. We gave 2 users owner permissions on the contact folder. When these 2 user try and add the shared contact folder to their outlook client, they get message "You do no have permission to view this contacts folder. Do you want to ask "MB owner" to share their contacts folder with you"? I have tried converting shared MB to regular MB and have the same issue. If I give the user full access to the shared/regular mailbox they can add the shared contact folder, but we just want them to have access to the contacts folder. I have tried this make the mailbox that store the shared contact an o365 only mailbox (regular and shared) and hybrid mailbox (shared and regular), but nothing works. The users that need to add the shared contact are hybrid users (AD/Exchange) and their mailbox is in o365.
Any ideas?
Thanks