I had a bit more time to dig into what happened but I can't really make it out. Some games that were previously marked as completed didn't get anything put in the progress field. Some got marked as in progress. Some were marked as completed, and after closing the Edit Game Details window (without making changes or clicking okay, just the red X), they got the green checkmark icon and started appearing in my Completed Games playlist and when I filtered games by progress Completed.
If I restore from a previous backup, the games affected are all marked as completed.
I've attached two platform.xml files; the first one from my latest backup before the new beta (on 5-18), and a manual one I created today after installing the beta, once I realized I was having the problem.
5-18-2025 Nintendo Entertainment System.xml
5-26-2025 Nintendo Entertainment System.xml