When a differential revision contains multi-line property change, then arc patch ... command sets new SVN property value regardless of existing value.

Instead it should:

1. take old and new property value from differential revision and create patch file
2. take property value from working copy and put it in temp file
3. apply that patch to that temp file
4. set contents of temp file as new property value to working copy

Relevant code piece: https://secure.phabricator.com/diffusion/ARC/browse/master/src/workflow/ArcanistPatchWorkflow.php;721bdf424bf33692ffe338b67e38072b742c92bb$624-631

P.S.
Existing implementation works flawlessly for single-line SVN properties.