That's pretty bad actually. If they do it on purpose, that's the wrong approach as it would randomly break any client that sync many files. Much better to do like Dropbox or OneDrive and throttle the connection as at least it means sync can be completed.
And if it's a bug, it looks bad because downloading and uploading files is the core feature of Nextcloud (or should be) and once again they don't seem to be getting this right.