Looking at the new Pending Sync Activity "metadata" was getting updated, then all those resynced photos began resyncing again this time showing "rendering" as well as "metadata". I was watching the “Syncing xxx photos” go down to 34 (from total of 7125 synced photos) and Pending Sync Activity said it was metadata being uploaded but after 40 minutes it’s “Syncing 6 345 photos” and Pending Sync Activity show rendering and metadata.
To me it seems like after the update to LR CC 2015.6 the Camera Profile for the synced photos might get changed, which then triggers the re-syncing.
I can’t remember for 100 % sure if I really have been having a different Camera Profile for the couple of photos I looked at but it does look suspicious as with one of my Nikons (D300) my import preset include changing to Camera Profile to Camera Standard v4 and it’s been changed to Camera Standard (the one without v4 suffix).If this is true (updating to LR 2016.6 will change Camera Profiles on photos) it’s alarming as Camera Profiles can change the look of the photo dramatically. It basically means all photos should be reprocessed to check the Camera Profile setting.
OS X 10.11.5 Activity Monitor says Lightroom is taking up between 200 % – 600 % of processor load and all available memory on my Early 2011 MacBook Pro with quadcore i7 2.3 GHz and 16 GB memory (first the memory went from 4 GB slowly to 8 GB continuing up to 10–11 GB before suddenly going to 5 GB and it's slowly getting bigger as I write this).
I have to stop working and let it upload those already synced photos, again. This happened with previous Lightroom update as well. I'm hoping it wouldn't happen with every LR update and will try to provide any info necessary to help track this issue down.