I am using Ardour 8.2 on Ubuntu Budgie 22.4 and have tried the file button example. In Cabbage 2.9.156 it works normally but after the VST2 export and loading in Ardour no file browser opens.
Maybe it has something to do with this: https://discourse.ardour.org/t/file-browser-windows-not-opening-in-plugins/107663. If I understand correctly, Ardour blocks the file browser because it is not realtime save. However, a workaround is also described there.
I have also tried it with Ardour under Windows but everything works normally there.