Why doesn’t deleting mailbox items in exchange reflect in available database white space

In Exchange 2016, I'm currently tasked with retaining ALL email items in Exchange… forever… (a discussion for another time)…

To do this, I set all mailboxes to litigation hold to prevent any user deletions. Periodically, I run a query to get the total size of all Recoverable Items for all mailboxes to determine the largest Recoverable Items/Dumpsters.

For a batch of the top users, I remove the litigation hold with:

Set-Mailbox {username} -LitigationHoldEnabled $false

and receive the following standard warning:

WARNING: The hold setting may take up to 60 minutes to take effect.

I then wait at least 65 minutes for the removal of the hold to fully take effect. (Previously, when I didn't wait the full hour, the delete operation wouldn't delete all that I expected, I learned my lesson there…)

I export the Recoverable Items/Dumpster items with:

New-MailboxExportRequest -mailbox {username} -filepath {path to a file} -IncludeFolders "Recoverable Items"

Which I then store off site.

Then I actually perform the deletion of the Recoverable Items/Dumpster with:

Search-Mailbox -Identity {username} -SearchDumpsterOnly -DeleteContent

When that's complete, I re-enable litigation hold:

Set-Mailbox {username} -LitigationHoldEnabled $true -LitigationHoldDuration 1095

and get the standard warning again:

WARNING: The hold setting may take up to 60 minutes to take effect.

My question is this, after I delete the Recoverable Items/Dumpster, why doesn't the "AvailableNewMailboxSpace" (white space) for my mailbox database eventually increase by as much as I've deleted?

To be clear, I am not asking why my database file size doesn't shrink when I delete exchange items, I am aware of defragging/moving mailbox databases to do this.

Usually after the delete of the dumpster items I do see a good chunk of white space come back, but it does not total the amount of space the deleted items used (the amount of space the -DeleteContent operation indicates was deleted).

Also, I've noticed that exchange seems to delete these items over time, since the AvailableNewMailboxSpace grows, slowly, even after the deletion operations. However, even two weeks later, it still hasn't recovered space for all that I have deleted.

i.e. I deleted about 20 GB of Recoverable Items/Dumpster and two weeks later the AvailableNewMailboxSpace has only increased by about 4.5 GB. I have not received new mail totaling 15.5 GB which would account for the difference.

Since the AvailableNewMailboxSpace seems to increase slowly after my delete operation, I assume there is some DB maintenance going on behind the scenes actually deleting items marked for deletion, but it's troubling that I don't eventually see all the space available.

Can deleted items still take up white space? Perhaps turning litigation hold back on, so soon, while the DB maintenance process is deleting is stopping deletions? Can I force DB maintenance to happen before I re-enable litigation hold?

Any insight would be greatly appreciated!