Thunderbird uses limed message fatching and in the case of some servers like google utilizes push notifications as well.

Push notifications have sever limitations on the number of folders they work with and can only be sort of relied on for mail in the inbox.

Times mail fetch will occur in the time frame in your account settings. But not all mail folders are polled for mail. Specifically the trash/deleted and junk/spam folders are not polled for new mail unless you select them. ( right clicking the folder and selection properties allows for a folder to be included in the fetch if it is not.) Having said that various things can see a folder deselected, so check they are included in the folders to be polled for new mail.

I have seen new mail slowed or dropped if it is subject to mail scanners from antivirus programs. So I personally do not recommend email scanning, nor do I use it myself.

I'm only talking about new messages in the INBOX. I tried what you suggested anyways: 1. I disabled scanning of email and browser activity. 2. I added all non-INBOX folders to the scan.

No change. Just this morning (June 20, 11:00AM), I have received at least 15 email messages dated and timestamped yesterday (June 19, before 1:00PM).

To reiterate, I never had this problem on any PC with the same email accounts and the same network settings. Until Thunderbird upgraded to the latest version, I had ZERO problems on this PC. To blame any other source for the problem is just unfathomable. The only change is the version of Thunderbird, therefore, the only possible source of the problem is, you guessed it - Thunderbird.

Where can I file an actual bug report for this problem?

before you file a bug report, I suggest you create a log of the process. Because I find in unfathomable that an IMAP server would not offer any new mail to be downloaded.

You will need to use GEKO logging of IMAP in V115. See https://wiki.mozilla.org/MailNews:Logging

A bug can be reported here https://bugzilla.mozilla.org/home but should be supported with evidence of what is happening. Developers need to reproduce things to test their fix. So determining the cause is often more important than the fix.

I also suggest you examine the headers of the messages. The date header is added at the time of submission to the SMTP server and really has nothing to do with the delivery time.

Also make sure accounts are set to get new messages no more frequently than 15 minutes. You might have had this set to things like 2 minutes in the past, but 15 minutes eliminates issues with servers still connected. It is a fairly standard diagnostic.

I have no idea what you mean when you say you added all non inbox folders to the scan> Personally I would just remove the antivirus. Windows has one that is perfectly adequate for most things and it needs to be eliminated as a cause.