Does this sort of match to https://bugzilla.mozilla.org/show_bug.cgi?id=1891687#c4 ?

Sorry, no. As I said, deleting, moving or renamimg a folder in the TB Icloud account will result in all the subfolder being Unsubscribed. I then have to right click on the account, select Subscribe and then put a tick back in all the boxes for the sub folders.

Hang on, you take a folder, say oldMessages and you rename it. What does the iPlatform do with a rename? Not all IMAP servers are equal. Some support rename, some do not those that do not delete the folder and add a new one.

Your description of the sub folders being unsubscribed sounds to me like that is what the iPlatform is doing, deleting the folder so none of the sub folders exist for a time until it adds it again. But I do not work for Apple and really have little interest in the internal working of their proprietary platforms.

This link indicates that the issue is the iPlatform thinking a rename is a remove.

I think the issue is probably not Thunderbird.

Surely the Subscribe is a Thunderbird thing, i.e. I can ask TB to only look at certain folders by removing the tick in the right click account Subscribe dialogue menu? The Icloud servers have know knowledge that I have done that, all they will see is a request from TB asking for the latest sync info from the folders that ARE ticked in TB. How can the Icloud server tell TB to Unsubscribe all my folders?