The button is only displayed when browsers fire the beforeinstallprompt event to signal that they support triggering PWA installation from custom UI elements, which is best/standard practice.
If it isn’t displaying, it typically implies the button wouldn’t work anyway.
The samsung galaxy phones should support the event, either with samsung internet browser or chrome browsers.
The users feedback is that, looking at their screenshots, the darker blue background that would contain the install button appears to be hidden behind the grey background footer menu on some devices, but not on others (as per first screenshot).
I'll follow-up with the grey background footer menu addon author.