When you move a message from one folder to another, Thunderbird "remembers" where you moved it to, and next time you start a move, offers to repeat the previous action. In this case, it thinks the last move you made was into an Inbox folder.

To mark a message as unread, I would do one of these:

• click the grey dot in the Read column in the message list (see attached picture) - this dot will be green for an unread message;
• use right-click then Mark|As Unread;
• via the menu, Message|Mark|As Unread.

All of these methods offer the ability to toggle between Read and Unread.

In regard to my picture, you may have different icons. These vary between versions of Thunderbird, operating systems and themes.

thanks for the solution slgu