The bug
In frozen ocean biomes, polar bears from spawners only spawn on ice (as oppose to grass for other biomes) at light levels of 12 or above.
However, ice melts at at light level 11, meaning that sustained polar bear spawning cannot be maintained in frozen ocean biomes.
In addition, polar bears naturally spawn in light levels of 7 or above, as well as other non hostile mobs spawning fine at light levels 7+, so the light level needing to be 12 for polar bear spawners appears odd.
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Issue appears to be then that polar bears from spawners do not spawn on grass in frozen ocean biomes, but instead only spawn on ice. I have attached screenshots as such, showing spawner on grass spinning indefintely, and a spawner on ice, having spawned a polar bear and then turning back into a pig spawner.
I cannot reproduce this in 1.21:
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Could have possibly been fixed in 23w45a alongside MC-131646, but that is speculation.
Spawners have different mechanics than regular spawning.
For spawners, polar bears require grass to spawn on.