Minecraft Username: CoolManPlayzMc Brief Description: 2 mobs stack to a 3x mob instead of a 2x mob. Instructions: Either use a spawner or find 2 mobs of the same kind, let them stack, they'll be 3x instead of 2x, it's the same for 3 or 4 mobs, they stack 1 extra. How many times did you recreate this?: Around 3 times. Result: Mobs stack to 3x. Expected Result: They stack only to 2x. Evidence:
Are you sure the spawner didn't just spawn in another polar bear? Your video doesn't show the spawners so it's hard to tell when they go off.
I've just tested this. It's just as @Videowiz92 said. Bug is absolutely invalid. You're just not seeing things right. @CoolManPlayzMc