Wow, I just associated my Logitech K480 Bluetooth Keyboard from SailfishX.

It works.
When some text input is required (that is: when the cursor is on) and the keyboard is on, the virtual keyboard disappears.

Note that in my Settings I have Language "English US" and Text Input / Active keyboard "Italian", Text Input / Keyboards "Italiano", Text Input / Active Layout "Italiano" (my K480 has an Italian layout).