I'm on a MAC. Thunderbird is 128.2.0ers. No firewall nor VPN. When the email comes in the MAX shows it in the top right corner.

It was gotten bad that I have to read my iPhone and iPad to read my emails.

Is this the issue: ignore thread flag. WHERE IS IT?

Perhaps you might try the knowledge base for information on Threading. https://support.mozilla.org/en-US/kb/message-threading-thunderbird#

The following didn't do anything:

Type mailnews.default_view_flags into the Search preference name search box.

Not sure about threading. The incoming emails ate not related.

I have seen that when the notification arrives on the top right of the display, and I click on it, I can read it it and then it is my in box.

After a wile they show up; maybe 10-20 min., and it helps to check on the total address and then on the inbox.

You do realize that Thunderbird only checks mail based on a time thing around every 15 minutes based on your settings in the account settings. But I would expect the messages to appear as soon as you are notified of them.

I have heard of a few update glitches where you need to change folders to another in the folder pane and back again to force a list refresh however. Could that be it?