Did you or have you noticed the exclamation mark in your first image, in the film strip?
That ! mark is there because LR want to either Read the edit data from the XMP or Write new data to the XMP. I suggest you Read from disk and see what happens. There are other images with that ! mark.
You can also select that adjustment pin and re-adjust the settings for it.
Looking at your other SS, the one from LR 5, there is that same ! mark
The exclamation mark is just from the metadata from having just installed this new version and has nothing to do with the edits. I did fix that, hoping maybe that was it, but it just adjusted the metadata. I suppose I could readjust the settings but I'd have to do that for all of my 100,000+ images and I'm not really up for that.
Since you suspect the problem is XMP files, let me ask you a question
Did you re-import the photos into LR 6?
I think so. They are all synchronized. In looking again this morning, the ones I said weren't edited. That isn't exactly true. Every image already has an xmp file because I have a custom preset I apply on import. The only thing I can think of that is different, is that the images with a problem I have done "edit in" Photoshop. Would that affect anything? I know it shouldn't, just grabbing at straws trying to come up with something since Adobe clearly has no idea.
You need to upgrade your Lightroom 5 catalog to Lightroom 6. In Lightroom 6, use the menu command File->Open Catalog and point to the Lightroom 5 catalog file.
If you imported or synchronized the files into Lightroom 6, you have done the wrong thing.
No, you're right. We did do that. I had to do that when I first started LR6. I went in and had to select a recent catalog. We also exported just one session as a test catalog to mess with, and so far no luck.