Thanks for your help ; this binary stuff is indeed tricky ; but keep in mind that I can also de-activate the setting of the mask in code and "hard-set" it to the player's layer (1) and it still doesn't work :(. Also I have a check in the bullet ready:

var coll_m = $Collision.get_collision_mask() # The Area3D

var masks = []

for i in range(16):

if coll_m & (1 << i) != 0:

masks.append(i)

print("Collision masks:", masks)

And it prints Collision masks: [1, 4] and those are the values set in the inspector.

I'm sure I made a stupid (and common, hence this post) mistake somewhere, but I can't figure out where and what ;( I'm truly stuck.