Pencil tool draws nothing on canvas (self-answered: disable Lock Alpha)

Hello, I just had an issue with the Pencil tool drawing nothing on canvas (while lines, rectangles, etc. worked) and wanted to share the solution.

I had Lock Alpha still enabled from a previous project (apparently the setting crosses projects). Setting is besides the brush size and angle:


I reverted it to Simple Ink and it worked.

I hope I picked the right keywords in title to help other people find the solution.