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.