I've been having problems saving metadata under certain conditions in 2015.8.. LR will say saving metadata, but the badge will come back on after.
So far, I've been able to narrow down that it seems to happen most often when an adjustment brush is used along with guided upright.
The problem comes in when I try to replicate it in photos that are ok with it. On problem photos - copying, then deleting the problem adjustment brush will allow it to save ok. But if I re-paste that copied brush, it wont save again. Even more puzzling is that if that if there are more than one brush, then it seems to only have a problem with ONE of them. And again, I haven't been able to determine a pattern as of yet.. It took me quite a while to even narrow it down to an adjustment brush issue, recently.
Also of note is that it will have problems saving even if there are snapshots with the offending brush, even if the 'current' doesn't include the problem brush.
Also important here, is that just turning off upright won't fix it - only removing the offending brush will..
Sooo, therefore it might not even be a problem collision with guided upright! ... It's just that I notice the problem occurs most often when guided upright is involved.
The only way I've been able to work around this is to create virtual copies with the offending brush (and the other settings). However, then I lose the belt-and-suspenders backing up of settings - which I find very important to me.