corrupt brush tool - light squares appear when using brush

Hi there,

Can anyone help me with this issue I am getting. Sometime when I am trying to brush light onto my subject, it paints in faded squares ( I've attached the screenshot, face blurred for privacy ).
These faded squares appear when I paint inside the face. 
It´s only when I restart photoshop that it fixes itself but it still does it quite frequently and it´s a bit annoying having to restart photoshop over and over again. I haven´t changed the brush settings and I'm wondering if anyone else has had this issue or knows what going on.

The program is Photoshop CC 21.1.3 working on a macbook pro 16" 2020. 

Thank you!