Exchange retention policies not working on some mailboxes

The Issue: We applied a retention policy 4 months ago that will clean up the deleted items folder by deleting anything older than 90 days. At 90 days, we saw many people's deleted items folders were cleaned up. However, we have a few mailboxes that seem to be more stubborn.

Troubleshooting: I picked one mailbox to troubleshoot against. I verified that the retention policy was applied to the mailbox.

I've increased logging in Exchange to monitor the Mailbox Assistants (ELC Library, Email_Lifecycle_Assistant, Service)

then restarted the Mailbox Assistant service, and ran Start-ManagedFolderAssistant against the mailbox and watched the logs. It processes the mailbox without issue.

I moved the mailbox to a different DB on a different server and applied the same steps. Finally, I ran New-MailboxRepairRequest against the mailbox, which comes back clean.

Yet, in his Deleted Items, we still see many messages like this, that say they are expired

EDIT: The only thing I didn't already try from the answer below's technet thread was Export-MailboxDiagnosticLogs. I wasn't aware of this cmdlet and will be checking it out later today. The Technet thread mentioned in the below answer, for clarity: https://social.technet.microsoft.com/Forums/en-US/8d13e08d-b3ff-45ec-adf1-52a2c88e5482/retention-policy-stamping-items-but-not-deleting?forum=exchangesvrgeneral

I did find something I believe to be relevant; when I export the mailbox in question using

New-MailboxExportRequest -Mailbox dh -IncludeFolders "#DeletedItems#" -ContentFilter {(Expires -lt '3/21/2019')} -ExcludeDumpster -FilePath \\path\dh.pst

the PST that's generated is empty. But if I run

New-MailboxExportRequest -Mailbox dh -IncludeFolders "#DeletedItems#" -ContentFilter {(Received -lt "12/15/2018")} -ExcludeDumpster -FilePath \\path\dh2.pst

the PST is over 300 MB and has the thousands of items he sees in his Deleted Items folder, the vast majority of which are years old and have been in his deleted items folder for years. The server must think these messages are not expired, based on this, which makes the behavior make sense, but just not the why.

Request: Are there other steps I can take that I've overlooked? My last step would be to export the mailbox, make a new one, and import the mail. Googling is made difficult by the massive number of people that think retention policies aren't working because they don't realize you have to wait the set number of days from when a policy is initially applied to a mailbox.