Edit I cannot find in Gitlab closed issue #7740 it was suggested that there was a fix needed for “use over metered network”. The “unmetered” aspect is mentioned in the contributions of @tcecyk in e/OS Drive/cloud sync not reliable.
My sync is ok, my Wi-Fi is known unmetered. I have toggled off System > Developer options > Mobile data always active.
As the sync not a fast job I can envisage this “active” setting allowing shifting of connection during the “check flags” then sync process.