I'm no casting guru(Shocking!) Could just be impurities in your lead. Your mold is square and true clean your heat looks even so that leaves the material that's the issue. Course I've only be messing with handloads for 39 years.
I would say try a little hotter on your lead. I like mine a little bit frosty. The powder coat seems to stick better for me. With the little 22 cal bullets it seems to help to run them a little hotter. Btw, 19 gr of varget gets me 1.5” groups with 60 gr gas check bullets.