old mailbox removed after license reassignment

Hi

We recently moved to security group based license assignment from direct assignment for our organization and during the migration around 20+ users lost their old mailbox and got a new mailbox provisioned. These are critical users in the company and we cannot retrieve their old mailbox. No mails in their deleted or recovery folders and the mailbox size is less than 1%. they only have mails available after the time we switched the license. Please note that this was a bulk migration and we migrated 120K users to this new assignment type. MS support has not gotten back to us on this. Has anyone experienced this behavior? if so how did you fix it?

Thanks a lot for all your helpful suggestions.