Does the same thing happen when you upload an image to your personal gallery?
Yes. I just uploaded an image to my personal gallery with CC-BY-NC-SA selected. When it finished uploading, the view page showed the license as CC-BY-NC, without the SA again. But I was able to edit the license after upload and the changes stuck, so now the license correctly reads CC-BY-NC-SA.
For a lark, I went back to the two game gallery submissions I mentioned in the top level post and tried to edit them again. The changes stuck this time. Huh.
By the way, today's attempt was with Firefox 126.96.36.199 under Win98. The TLP's attempts were under Win2K with Firefox 3.0.1.