Kirk,
I had it happen again this morning.
I was in a contact Sheet for a folder. The images are cataloged and the catalog was active. I used the drop down to view only the Tagged files. This was a mix of JPGs, NEFs, PSDs and DNG. Those different file types are different file formats of teh same images and I had the files sorted by file name so that the JPG and corresponding other formats were next to each other.
I brought up the Metadata (IPTC) dialog for the first image, a JPG, edited it, copied it, clicked the save and go to next file button (the corresponding NEF), clicked the Paste button to enter the copied Metadata, then clicked the Save and move to next file button. The image I had updated originally had about 2 lines worth of keywords, after editing it had 5.
I was now at the JPG for the second image, which also had a corresponding NEF. The Metadata dialog showed only the original keywords for this image, about 2 and a half lines worth. I added additional keywords via the Structured Keyword dialog (I almost always use this for keyboarding), then repeated the same procedure as above to update and save the IPTC info to the JPG and NEF for this image. Again, there were about 5 lines worth of keywords.
When I moved to the Metadata dialog for the 3rd image, also a JPG, the 2 existing lines of keywords were shown, but the last 3 lines from the previous image were also shown. Closing the Metadata dialog and reopening it reset the screen to show only the keywords for that image.
Maybe this will help you track down this issue.
On a scale of 1 to 10, with 1 being annoying but doesn't really affect anything and 10 being I hate your software and a mob is coming to Portland to burn down your office, this is barely a 1.