This is a long standing problem that I've wanted to report before, but I don't know how to reproduce it consistently. However, I'm not seeing a difference in behaviour compared to earlier versions. The file dialogs will filter correctly for a while, but they will eventually start showing files other than .json. I've seen it both for the save and load dialogs. Restarting VAM fixes it.
This isn't a new problem, but it is a rather annoying one.