Who is the account provider?

I don't understand the logic for manually moving a message into "Sent". Personally I'm much more likely to be moving a message out of Sent to a folder where the message can be filed alongside related correspondence. It seems crazy to me to keep a string of correspondence in different folders. So I strive to keep Inbox, Sent and Trash empty and I file correspondence that's worth keeping in its own folders.

But apart from that, my gut feeling about Sent is that it is a system folder and is managed by the email client. I don't think it should actively refuse messages being put there by the user, but all the same, this is opposite to the normal direction of flow.

I asked about the the provider because if you have an IMAP connected account, you're also affected by the server that holds your account. Some providers have funny rules that interfere with message filing. Gmail in particular suppress copies to self because in their view, such a message would be a duplication of one already in your Sent folder. I don't know if this means they actively monitor what goes into Sent.

Practically, Thunderbird does occasionally suffer instances of dropping things while being dragged. You can avoid this by using a right-click and "Move to" or "Copy to".

And go the folder where the search says it found the message, right-click and select Properties then Repair Folder. It may just reappear.

Thank you for your response. Yes, I did try repairing the folder but that did not restore my email.

Lux Scientae is the account provider. It is an IMAP account.

Yes, I do keep related coorespondence in separate folders. This email got copied to one of those folders due to a misconfigured message filter. When I tried to move it to where it should have gone, that's when it got lost.