As discussed in
PICARD-1858 builds with any PyQt version >= 5.13.2 (with the latest version tested being current 5.15.1) do not run on macOS 11 Big Sur.
On macOS Big Sur the application stops responding after start without showing the UI. Actually even a very minimalistic Qt5 app with only an empty window will stop responding as soon as the app's mainloop gets executed. The exact reason is unknown, but it happens with PyQt >= 5.13.2 (tested up to 5.15.1).
This actually only happens when running from the packaged app. Running from Picard from source works fine. For now I can't say whether the core problem lies within PyQt5 or PyInstaller.
For the future we should be able to upgrade to make use of improvements and bugfixes in Qt5. I expect that there will be some macOS 11 specific improvements, e.g. there are open issues about theming improvements QTBUG-86513 and support for arm64 QTBUG-85279.