A WebDAV server is allowed to modify items during a PUT, in which case it won't return an eTag. SyncEvolution should treat the missing eTag as "server has updated data" and import that changed item in a second cycle.
This can be made more efficient with the prefer=return-representation header.
See also the behavior of the Apple Calendar server in Client::Sync::eds_contact::testOneWayFromLocal: it returns an etag in the response to POST, but then later seems to update the item (property reordering) and updates the etag.
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