Outlook not automatically downloading new email

Hoping someone has seen this issue before and know what the fix is. I have one user who’s mailbox (in cached mode) is not automatically downloading new emails in desktop Outlook client.  On initial launch, Outlook’s Inbox will be up-to-date, but afterward does not automatically download new emails anymore – except if manually clicking on the ‘Update Folder’ button in Send/Receive ribbon.

I tried just about everything I can think of, but none of the following worked:

Clicking on the ‘Send/Receive All’ button doesn’t work.  (Only the ‘Update Folder’ button will trigger download of new emails.)

Creating new send/receive group

Create new profile, new OST, different cache length

Switching to a brand new PC. (Other user mailbox on the same PC works fine.)

Removing throttling policy doesn’t work.

Reducing mailbox size & item count doesn’t work. (Inbox has 220 items, Calendar has 1,573 items.)

Setting end dates on all recurring meetings.

Moving mailbox to a different Exchange server/database.

Toggling ‘Work Offline’ button.

Disabled ALL add-ins
Added regkeys from https://learn.microsoft.com/en-us/outlook/troubleshoot/performance/performance-issues-if-too-many-items-or-folders

Online mode works ok…but Outlook is slow in online mode. Mailbox is hosted on Exchange 2019 servers on Windows 2019 server.  Win 10 with Outlook 365 MSO v2202 64-bit.  Classic Hybrid and using Modern Auth for Outlook.  Since this is happing only with one user, I guess it’s something in his mailbox config.

I could delete and re-create the mailbox, but user has recurring online (Team/Zoom) meetings that we don’t want to cancel/re-schedule.

Anyone has other any ideas or suggestions to try? I have an open Microsoft support case for the past 2 months but issue persists.