Apple Mail 16.0: random emails not loading

This is how I have my Privacy preferences set:

I do not have a problem with emails not loading. Therefore give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Any AntiVirus software installed or this specific ad blocker Ad Guard

Not too important at this time. They will show up the the suggestion by @ Old Toad to download the Application Etrecheck directly from the Developer.

If the problem went away in Safe Mode but came back when you rebooted normally there's some 3rd party software that may be causing the problem.

Therefore, download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can examine the report and see if we can determine the culprit.