Pico: strange behaviour with setWatch on BTN1

Hello,
I experience a reliability problem as exposed above. The setWatch function sometimes misses the button event (I press once a second). It's not a floating problem, my voltmeter says 3.21 V. Here is the code :

var ledjaunestate = false;
pinMode(B10, 'input_pullup');
setWatch(function() {
ledjaunestate = !ledjaunestate;
digitalWrite(B1, ledjaunestate);
}, B10, { repeat: true, debounce : 5, edge: "falling" });

It's on Pico with firmware : espruino_1v85.506_pico_1r3_wiznet.bin
Any clue ?

Thanks a lot
GB