Are you viewing in Local Folders or the account folders? If in Local Folders, you may have moved just the headers.

Thanks for the question, but no, I'm viewing it in "account folders." That's why I can see the texts on the server. I also have an Inbox in local folders, and none of them are on the server, and I looked at a few emails ans all the texs are there.

I am baffled by this. The only thing I can think of is to reinstall Thunderbird, but I have no way of knowing if that will work, so I am asking here first.

Please clarify: from your post, I infer that when you view in account folders in thunderbird that the text is there, but if you view in the Local Folders folders that the text is not there. True? If so, that implies that when you copied to local folders the account folders only had headers. IMAP doesn't populate content unless you click to read the messages. In that case, there is no need to reinstall. Once IMAP messages are copied out of the account folders, thunderbird no longer populates content there.

If not using 128.1.1, updating may help if what you're experiencing part of what may have been a bug. Also, try a folder repair afterwards and see if that helps.

I was writing you a long reply about how I had to explain things better, but then your second message mentioned the "Repair Folder" function, which I had not known about, and that seems to have worked, though I've only checked a few emails. Weirdly, it also offered a few dozen emails as if only just received, but they are all unimportant and I can just delete them. So, thanks a lot, and now I'll know what to do if this happens again, and after sending this I will mark the problem as solved.

Repair would have only resolved that one situation.

https://bugzilla.mozilla.org/show_bug.cgi?id=1890230#c105 has a potential fix if you want to try it.