I came across this issue whilst trying to enable UI navigation - the D-pad button's aren't buttons - at least as far as Construct is concerned. Instead you need to use gamepad.axis to detect if they're pressed.
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On all my gamepads (5 different makes) d-pad buttons are definitely buttons.
I imagine pressure sensitive d pad buttons could possibly be interpreted as axis. My own experience has similarly been that d pads are buttons though.
I submitted a bug report to Firefox, since PS5 controller is currently using Axis 9.
Be interesting to see if it gets fixed:
bugzilla.mozilla.org/show_bug.cgi