I undid the screw and checked the o ring, and that is fine. The screw hole is perfect (not stripped) and the screw threads are clean. There are no contaminants anywhere, so this is a mystery.
I would try to tighten the bleed screw more, but I accidentally overtightened the rear caliper bleed screw hole during this most recent brake bleed session.
Is there any explanation for this sphere of fluid at the top of the bleed screw despite it being tight?