@gduchateau2020 hello gduchateau and congrats on your decision to go o365! In the on-prem environment, can you check if there is still an object for the mailbox that was moved?
It sounds like after the migration, the mailuser object for the room mailbox may not have been created successfully.
Consider creating a mailUser object in your onprem environment and setting a targetAddress property to match the room mailbox SMTP address. This should allow it to show in the onprem GAL for non migrated users.