The case of a dissapearing button due to BitmapMask

I’m building a control panel with 2 buttons that is supposed to look like this:
I’m using a class to create these button, and there’s a BitmapMask applied to the top btn graphics to make it look it’s actually been depressed and moved down by about 8 pixels. These are the masks now made visible:
however, as soon as I apply BitmapMask to the button, ONLY one of the buttons shows on the scene!!

in my scene I create buttons by these 2 lines:
let left_btn : CharPushBtn = new CharPushBtn(this,this.gameCenterX - btnOffset,btnY,“left”);
let right_btn : CharPushBtn = new CharPushBtn(this,this.gameCenterX + btnOffset,btnY,“right”);

whole code is here:

To me this looks suspiciously like a bug in Phaser 3 (using version phaser-3.19.0), or can anyone spot I’m doing something wrong here??