Does the same happen when you run Thunderbird in "troubleshoot mode"? https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird

I just entered thunderbird in troubleshoot mode, and then asked it to compact folders, and the not read flags were not reset. That presumably means that one of my add-ons is causing the not read flags to get reset. In fact, the not read flags are not reset on my laptop in thunderbird, so maybe a comparison of the two can help with my problem.

Do you use Bitdefender and did you install the Bitdefender antispam toolbar in Thunderbird? Bitdefender toolbar is known to cause this issue https://community.bitdefender.com/en/discussion/89274/bitdefender-anti-spam-toolbar-causes-thunderbird-to-ark-all-incoming-emails-as-read?tab=all

No, I don't use Bitdefender. I use the windows built in security for anti virus and other protections. I also don't use the Bitdefender thunderbird add-on.

So Bitdefender is not the culprit. I cannot find the cause for you since the issue doesn't happen on my computer. Please read carefully the paragraph "The problem no longer happens in Troubleshoot Mode" https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird#w_the-problem-no-longer-happens-in-troubleshoot-mode

How do I contact the developer of the add-on that caused the problem? I see no email address for the author of the full address column add-on.

The developer of which add-on?

The Full Address Column add-on.

Are you sure that the Full Address Column add-on is causing the issue with your unread flags? Which would mean that if you disable this add-on the "unread" icon is displayed correctly?

I just know that I enabled the add-on and had new email and compacted folders and the new email was no longer marked as unread.

Unfortunately I am not able to find a solution for your problem.