Dished top has extremely coarse steps

[PWBarrett]

I've tried spherical, old spherical, and cylindrical, on dsa and dcs keycaps, and in every case the top ends up with maybe 20 steps, almost like it was trying to be some coarse non-slip texture. This also makes the embossed text almost unreadable. Looking at the model in the OpenSCAD designer, it looks fine, but looking at the generated STL, the steps are there.
Is there any way to smooth this out?

[PWBarrett]

Tried playing with parameters like $fn, $fa, $fs and it didn't help.
Maybe this is just an issue with OpenSCAD's STL output?