If you create a basic TextInput, assign som arbitrary text and call selectAll() on it you will see that the selection is cleared as soon as you give it input focus and the keyboard shows up.
It happens even if you call selectAll or select while the keyboard is visible.
Note that GBoard is the new name for the official Google keyboard used by Android One, Google Pixel and potentially up to 50% of current android users. You can install it on any android phone through Google Play.
Seems to affect all released versions of Qt. I suspect this is because the GBoard has a built-in text selection functionality. (accessible by clicking the G, ".." and "Text editing")