Odd results with Pathfinder

I’m getting odd artifacts when using pathfinder where the overlapping objects meet tangentially at an edge. I’m also going to try Shape Builder to see if I can get different results.

[edit] Same thing happens with shape builder.

Below, two circles  - I’m trying to remove the yellow from the red; the highlighted thing on the right is the closeup of where they meet at the top. The paths, however, look fine.

Any idea what might be causing this?

thanks