alright, that’s interesting. never happened to me before.
so… i played around with settings of brushes and i must say it behaves much more unpredictably than i thought. i’ve tried different brush settings, different save settings and sometimes i got really weird results like painting with white when mask was actually selected, while white wasn’t an alpha colour (or even present in palette) and the sprite wasn’t in index mode anyway. it’s really strange and now i question my understanding of saving brushes and the reality itself 8O
however, i managed to recreate your issue and it seems like this happens because you have a colour stored with your brush and you try to re-select the colour you already have set as foreground colour. when you ctrl+click and pick up different colour, the brush should start to accept input from colour picker.