Emails not syncing past a specific date on all iOS devices

Question

My Gmail account is not syncing emails past a specific date (Aug 24) to third party email clients on my two iOS devices. This is the case in both the Mail, and Outlook email clients. The Gmail app is working fine.
Steps taken to try and resolve include: Resetting the account in Outlook; removing and re-adding Google account to both Outlook and iOS Accounts; deleting and re-installing the Outlook and Mail apps; restarting devices between re-installs and adding accounts. 
Results: In all of the above cases, emails re-synced up until Aug 24th. No new emails are being downloaded, and the Outlook app appears to hang on 'Updating...' when trying to force a re-sync after all prior emails have been downloaded.
Only thing that hasn't been done is a full device reset - however, given that the same issue exists between iPad and iPhone, it seems more like an Account issue. My only other thought is potentially doing a password change, but it that doesn't jive with the fact that the account can authenticate and sync emails to a specific date.
Any thoughts or advice welcome. Thank you!

David Barkhausen · Accepted Answer

Marking response provided by Craig S 1374 as solution, but should clarify that it was actually a comment in their linked issue that has the resolution, specifically this thread: https://support.google.com/mail/thread/71739044?hl=en&msgid=71739044
Suspected Root Cause:
In summary, Google considers that emails encrypted using S/MIME in Gmail is compatible only with GSuite accounts, not free Gmail accounts. So basically, if someone sends you an encrypted email in this format, then it creates a block when trying to sync messages using IMAP to third-party email apps/clients (e.g. Outlook or Apple Mail).
Recommended Fix:
For those with a similar issue, I recommend going through all the emails received on the day that messages stopped sync'ing to Outlook/Apple Mail and figure out which message might be encrypted and thus blocking all other messages. Locate the message in Gmail (web or mobile version) and either delete it or move it to a folder that you aren't sync'ing through IMAP. If you can, delete/archive all messages received on that date - see if that solves this issue.
In my case, the offending email  was an encrypted bank statement. Once deleted, emails started sync'ing to the current date in Outlook on iOS devices.

Craig S 1374 · Accepted Answer

I faced the same problem, but managed to track down the cause in my case.  See https://support.google.com/mail/thread/70918295?hl=en to see if it helps any of you
Google definitely has a bug on there end, but continues to ignore it.  They clearly don't care about their users.
If you haven't already, I also recommend opening a support case with Apple.  I've provided all the details to them as well and been told that they won't implement better error handling in their mail application unless they get enough reports of a problem.