When you call removeAll() and sync() on a Store (appendId enabled) with 1 record, the Rest Proxy will fire a DELETE, but won't append the Id. It will however send a body containing the record. Sending a body on a DELETE request looks strange to me (and Fiddler .
The Docs contain an example showing the expected behaviour is appending an Id to the url.
This behaviour does not occur when deleting a specific record by calling remove(record)