So this is something I only noticed when I moved to a new phone. The option in the settings to add Piepmatz to "open with" no longer works, probably since the introduction of Sailjail. When I launch Piepmatz through the terminal then changing that option does work. And after you add Piepmatz to "open with" it will keep working even with the sandboxing, so only enabling or disabling got broken.
So this is something I only noticed when I moved to a new phone. The option in the settings to add Piepmatz to "open with" no longer works, probably since the introduction of Sailjail. When I launch Piepmatz through the terminal then changing that option does work. And after you add Piepmatz to "open with" it will keep working even with the sandboxing, so only enabling or disabling got broken.