Welcome!

Like sharpshooter said, the actual thumb safety is slightly out of adjustment. Simple enough to drill the plastic ball as said, exposing the set screw. Reinstalling the safety blade is easy enough & I do recommend it. Without it, a follow fire after closing the bolt is always possible. Although, it’s possible with any single-stage trigger.

This picture shows the set screw sticking out the top. The plastic can’t be seen on the bottom.