Meta query and compare “!=” not working as expected

I realize that maybe this could be an already submitted question and I searched for it, but couldn’t find it.

I’ve got the following simple meta query:

What I expect is to get all the “custom_post” posts in the relative Archive page that DOESN’T have the “is_hidden” checkbox active (so an ”/empty value instead of ‘on’ -> this is the value I get from CMB2 custom field checkbox). But it doesn’t work, as the result is no posts at all.

While if I set the ‘compare’ to “equal” (‘=’) it works – meaning: I get only all the posts with the “is_hidden” checkbox set to ‘on’.

Maybe I’m missing something…