Hello Sid-0195,
According to the situation you described, the problem occurred after you migrated the mailbox database and you could not see the availability of some attendees. So I would like to know the difference between those attendees whose availability you can see and those who cannot, such as are they located in the same database? Are the attendees in question located in a specific database? If possible, it is recommended that you check first and you may try to move one of the affected attendees’ mailboxes to another mailbox database or a new created database to check the result.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.