i think i did have a similar issue, fixing permissions seems to have taken care of it for me....
since upgrading to high sierra i have ru into several issues related to permissions, and now apple has limited access to fixing things (cant change permissions to root anymore)
not sure who is to blame, usually we, the consumer are stuck in the middle in situations like this....but i am pretty sure that adobe is already working on an upgrade to LR CC, it scares me a little to think about how many niggles have to be ironed out now that the release is out in the wild and people actually are using it......
did you ever find a solution to this problem? I am knew to Mac and I don't really know my way around yet.
From Lr Library select thumbnail and use Cmd+E
Edit in Photoshop and use Cmd+S to save
Press Cmd+W to close in Photoshop and send back to Lightroom
N.B you must use save or Cmd+S, not “Save As” as that will create a new document which Lightroom will know nothing about.