Hello,
I have typical Scenario before you.
There are two users X and Y.
For User X I created a remote mailbox using the command enable-remotemailbox and the creation was successful and the user is able to access the mailbox and do the general stuff. The user X requested access for a shared mailbox in on premise and I gave access to the shared mailbox. But the User X is not able to open the mailbox.
User Y has an on premise mailbox and we migrated the user Y to cloud and later assigned user Y access to the same shared mailbox as User X and he was able to access it without any problem (full access)
Any one seen such scenario.