android hardware back button does not work in kivy

[Fak3]

kivy version: git master (and latest stable)
p4a version: develop branch (and latest stable)
backend: sdl2

i have added keyboard hook for hardware button as suggested in p4a documentation:

class YourApp(App):

   def build(self):
      Window.bind(on_keyboard=self.key_input)
      return Widget() # your root widget here as normal

   def key_input(self, window, key, scancode, codepoint, modifier):
      if key == 27:
         print('back pressed')
      else:           # the key now does nothing
         return False

The hook does not receive any hardware backbutton input until i click any kivy TextInput. After i click any TextInput, android keyboard appears, then i can close it, and from now on hardware back button works just fine everywhere in the app (key 27 input is received). Another quirk: it stops working again if app is paused and resumed.
It happens only on some android devices: namely ZTE nubia z11 mini (android 5.1) and samsung galaxy S5. On some other devices it works fine.

[Fak3]

Fixed upstream: https://hg.libsdl.org/SDL/rev/b5cd5e1e4440
https://bugzilla.libsdl.org/show_bug.cgi?id=4702