Used Joplin Cloud for a while. Decided to clear down the client and the server and start again.
Joplin Desktop 2.1.7 was reverted to "unused" by way of a VM snapshot, so there was no residual data. Before syncing the client form the first time, the default notes were deleted from the client and a set of 24 test notes imported. Joplin was connected to the server and synced.
The test notes and resources were uploaded but Joplin then fetched 2569 items from a server that should have been empty apart from the notes, resources, tags , metadata etc. that had just been uploaded. 2569 is probably the number of items that were stored on the server before the data was deleted.
Nothing extra appeared in Joplin.
Compared to how long it took to sync the few new notes this "ghost" sync took a notable amount of time.
It would seem that despite all items being deleted on the server there is still data remaining that the server will "sync" with any new client. I suppose that this could be as a result of how the sync works rather than any bug. However if the server had previously been heavily used this could take quite a while as thousands of "non-items" are apparently fetched.