Outlook Cached Mode Repeatedly Re-syncs Mailbox After Restart (Starts Again Around 3.99 GB)

Hi everyone,

I’m experiencing a strange Outlook Cached Exchange Mode issue with a Microsoft 365 mailbox after a recent Windows rebuild and wanted to see if anyone has seen similar behavior.

Environment:

• Microsoft 365 mailbox (Exchange Online)
• Outlook for Microsoft 365
• Version 2605 Build 16.0.20026.20076 64-bit
• Windows 11 25H2
• Fresh OS rebuild performed twice
• New Outlook profile created
• Office completely reinstalled
• OST recreated multiple times

Issue:
When Cached Exchange Mode is enabled, Outlook starts downloading/synchronizing mail normally, and the OST file continues to grow correctly. However, after every reboot or Outlook restart, Outlook again shows “Downloading…” starting from around 3.99 GB.

Important observations:

• Online Mode works perfectly
• OUTLOOK.EXE closes properly after exit
• OST file is NOT deleted and continues growing
• Sync slider changes (1 month, 1 year, all mail) make no difference
• Disabling Outlook indexing did not help
• New Outlook profile did not help
• Reinstalling Office did not help
• Problem only started after OS rebuild
• Before rebuild, same mailbox worked normally in Cached Mode
  No pending office or windows update.

It does not appear to actually re-download the mailbox from scratch because the OST size keeps increasing, but Outlook repeatedly processes/downloads from around the same 3.99 GB point after restart.

Has anyone seen:

• Cached Mode replaying synchronization repeatedly after restart?
• Similar behavior on recent Current Channel builds?
• Security/EDR products interfering with OST synchronization state?
• Any known regressions with Outlook Version 2605 Build 16.0.20026.20076?

Any suggestions or similar experiences would be appreciated.