Pencil in Lightroom masking feature does not work anymore

Hi, 

I've always used the masking tool in Lightroom Classic and then using the pencil to define certain areas. Since a few days, the pencil is no longer working. No error message, just not doing it. Example: when I use a mask to select the background and the tool selects a few parts as part of the background that I want to exclude from the background. I would normally use the pencil to remove that part from the selection. When I now use the pencil, nothing happens. See screenshot of an image where you see selected red parts on the horse on the left side, which I want to remove. 

Can anyone help? 

I'm using Lightroom Classic 13.2 on a Macbook Air