Very often this kind of behaviour can be attributed to the server. It's most often in connection with junk (or spam) but let's see if the server is to blame here.

Try moving the message(s) to the Inbox folder in your Local Folders account. This Local Folders account is operated by Thunderbird and is totally independent of any server. So if the message stays put, we can safely say that the server, and not Thunderbird, is responsible for the message's unbidden movements.

Having made some more tests, it turns out that this behaviour only occurs when a message has been deleted in Thunderbird. If I log into the 1&1 Webdesk and delete a message from there, it can be moved back into the Inbox with no problem. Once a message has been deleted in Thunderbird, it is no longer possible to move it back to the Inbox by any means. It seems to have been indelibly marked as Trash! This problem occured on PC running Windows 7 and Thunderbird 60.3.3 and still occurs on 60.4.0. I have no such problem with a MacBookPro running Yosemite and Thunderbird (similar versions). I have the impression that it is Thunderbird which is causing the difficulty in some odd way.