Thunderbird Users –Ever Have Emails Vanish?

The first occasion:

I was waiting a response to an email sent via one of my accounts that uses Yandex Mail for Domains. As Thunderbird fetched email, I very briefly saw a subject line from that company flash past in the notification area at the bottom of the Thunderbird window.

However, when I looked, the message was not in my Inbox, nor had it been moved to the Spam folders or the Bin.

I logged onto Yandex webmail and checked there, in case it had been intercepted server-side and binned there. Same result. The email was nowhere to be seen. It was as if it never existed, apart from it was listed in the mail logs as having been received.

I contacted Yandex support and got a typical unhelpful copy/paste reply and thought not much more about it. As Yandex have proven themselves to be fairly incompetent in the past, I just put it down to experience and resolved not to use that email address for important stuff again.

Fast forward to today and the second occasion:

This time it happened with an email account that uses Google's mail-servers. Which would eliminate the 'Yandex factor'

As I usually do, I had a look through my Spam folder in Thunderbird, to check there weren't any false positives in there, before I emptied it. I spotted an important email that had been marked as spam. So I unmarked it as spam and dragged it into my Inbox. Similar result to last time. GUI showed the email dragging out of my Spam folder and into my Inbox but, when I looked in my Inbox, it wasn't there.

As with the Yandex account incident, I then checked the account online in a browser via Gmail and found the email was nowhere to be found. Not in any of the folders which sync to Thunderbird or in any of the server-side only ones.

Hence the question. Has anyone else had Thunderbird 'disappear' emails as if they had never existed, like this?