This is ... bug like.
The user moves a folder inside another, the resulting path exceeds the maximum length, the folder's content is no longer accessible, the user complains.
I don't think this is a bad idea. If this is really the issue.
Double trouble. The user proceeded to move the parent folder. Most subfolders moved as requested. Those whose path exceeded maxlength are stuck in the origin, and the full tree is no longer accessible: it is still there, you can see it, but the mail client says it was deleted.
I lost count of the number of times I had to rescue users out of this mess, by going in manually into dovecot's storage.
So...
+4. dovecot refuses to move folders if the resulting path exceeds the maximum length.
I think it is possible to specify different storage for a single user via the (special?) user db. Why don't you try this, with this option LAYOUT=index