Polar bears only spawn on ice; they cannot spawn on snow or grass. This is makes it extremely rare to see them in snowy tundra biomes.

Steps to reproduce

1. Open the test world, MCPE-105221 Polar Bear blocks.mcworld. It consists of a 4-chunk spawn platform in a void frozen ocean world. Command blocks in front of the player switch the platform between snow, ice, and grass. Repeating command blocks Mark polar bear spawns with purple concrete and kill everything to keep the mob caps open.
2. Set the platform to snow.
3. Wait a long time.
4. Repeat steps 2-3 for ice and grass.

Expected result

Polar bears spawn on snow, ice, and grass, or at least in snow and ice.

Actual result

Polar bears only spawn on ice.