Strays not spawning in Frozen Ocean or Snowy Beach Biomes

Strays are not spawning in frozen oceans, deep frozen oceans and snowy beaches, despite the following conditions being met:
Light Level 0 (night)
Sky Access
Y level between 60 and 66
Other skeletons spawning

Over the course of ~3 minutes, I observed 11 skeletons whose spawning met the following conditions, yet not a single stray. This is far above the necessary statistical significance, given the skeleton should have an 80% chance to be a stray.