I just figured it out. I'm not entirely sure my trick is doing anything at this point. It may have at one point... but who knows.
The trick is to not use native file dialogs.
Code:
options = QFileDialog.Options()
options |= QFileDialog.DontUseNativeDialog
fpath, _ = QFileDialog.getOpenFileName(
w,
"Select LanguageTool Java file",
"",
"languagetool-commandline.jar (*.jar)",
options=options
)
Unfortunately, that also means the dialog won't automatically match the system theme. So if you want that, you'd need to pull in the match_dark_palette portions of plugin_utils ayway.
Hope something there helps.