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
Open the test world,
. 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.
Set the platform to snow.
Wait a long time.
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.
This issue was fixed in one of the 1.16.100 betas prior to this report being created but I did not know that at the time.
In 1.16.100 polar bears now spawn on ice, snow, and grass in snowy tundras, and on ice-only in frozen oceans. It is evident that a change was made in the polar_bear.json spawn rules file, because there are now separate sections for ocean and !ocean biome tags.
This is consistent with the intention stated in the Aquatic Update announcement that spawning on ice-only in frozen oceans was necessary prevent mothers and cubs from being separated.