The syncing process simply writes files to the SDDM user's homedir; it doesn't do any of the fancy notifying that can happen when you change settings in a running session. As a result, old cached stuff can be kept around and leak into the user's settings.
Let's always clear the cache at the beginning of a sync to make sure that we return to a fresh clean state.
BUG: 440957 FIXED-IN: 5.24.6