A folder portal is by definition not operating directly on the desktop. Therefore it relies upon a path to the folder in question. If the path changes in any way, no matter how slight, because these are not self-healing (windows shortcuts can self-heal and there are utilities to assist with that system-wide that you can run to automate in batch,) you end up with non-existent folders and the portals show up empty.
You don't have to delete them, however, or "purge" anything - you just have to manually relink each folder portal to a new folder. This won't change the name of the folder portal but it should update your contents, since that is how it's supposed to work.