Lunar phase should be showing the moon's shadow (on itself) which will not be circular from our perspective on Earth except during the new moon phase.

This looks more like what is seen during a solar eclipse.

Yeah, this rendering is Just Wrong - moon phase is viewing a half-lit sphere from different angles, no external shadows are involved. Maybe look at https://github.com/chubin/pyphoon instead, which claims a history including Jef Poskanzer code going back to the 1970s...