Update: I have found that if I save the message to a file using TB 140.1.x and copy that file to a place where Thunderbird 115.x can load it, the message will be displayed correctly without corruption. So, I am guessing that something is happening that corrupts the message when the message is downloaded from the POP server. From observing the text of the corrupted versions of the message to the text of the UNCORRUPTED version, it looks like part of the header is being deleted when TB 115.x is downloading the file from the server. I can't even begin to imagine what could be causing this to happen.

You are using bitdefender?

If so, In Bitdefender -> Protection -> Online Threat Prevention -> toggle off the setting Encrypted web scan. Restart computer and Thunderbird.

Does that help?

I have made the changes you suggested and I am waiting to see what happens when the next commercial message comes in. I will update this when I see what happens.

After following your suggestion, I see that it hasn't fixed the problem. I do appreciate your attempt to help however. I'm still hoping for a solution at some point.

The problem does seem to be with Thunderbird 115.x downloading the messages. When I go to my webmail page on my mail service and make it download (push) the message directly to Thunderbird, it comes through intact and displays correctly. So there is something going on when Thunderbird logs into the mail server and tries to "pull" down all of the new messages.

Bitdefender is definitely having problems. It's not Thunderbird.

Please follow the issue at https://www.reddit.com/r/Thunderbird/comments/1mo693n/comment/n8ammvq/

While I am not 100% sure yet, I believe I have found the cause of the problem:

While rechecking server settings in TB, I discovered that the Connection Security had been set to "None" instead of "SSL/TLS". I have only received one email since correcting this but it came through correctly and displayed normally. While I cannot be sure "why" the server setting was changed and I don't remember even going into the server settings prior to this issue, I can't say with certainty that didn't somehow "fat-finger" it to the incorrect value. And again I can't be certain that changing it to the correct setting will correct the behavior that I was observing until I have received several more email messages that are multi-part mime encoding with rich content. I am keeping my fingers crossed and hoping that I have solved the problem and I will update this case once I have been able to determine with reasonable assurance that my solution is working. Many thanks to Wayne for his assistance.

Thanks for keeping us updated.

(Didn't realize I posted the same thing twice)

Will be interesting to learn if the TLS change is a permanent fix.

With a few correct email downloads without issues, I think I can say with reasonable confidence that the problem has been fixed with the server setting using the correct value for my email service. I will be marking this issue as solved.