Exchange Online Licensing / Remote Mailbox - Leaver Issue - Removing License

Wonder if anyone else has seen or addressed this issue before and how it's handled.

Exchange Online requires a license be applied to the user/account. Can be done in different ways but one is by Active Directory Group.
When a user leaves, Identity Manager disables the account and removes the groups and access from the user and their account.
This results in the Exchange License being dropped from the mailbox, which marks it as inactive and unlicensed.
When in this state, others are unable to be provided access to it, because it's unlicensed/inactive.

Need to retain the license so the Mailbox remains active so that others can gain access. Retaining the license is the hurdle.

How have people managed this scenario given that the access and license group is removed from the person/account and therefore retaining the license does not seem possible.
Does anyone have any recommendations / Solutions to this issue. 

Any help/info/suggestions would be appreciated.