Has been reported before in now closed MCPE-37971

Polar bears swimming under a 1 block thick ice layer (as found between structures in the Ice Spikes biome) can rear up and clip through the ice to cause damage to the player.

Duplication:

• Go onto a ice cap on a frozen ocean and find an area where polar bears are prevalent.
• Make a hole in the ice where it is 1 block thick (most of the frozen area).
• Annoy a polar bear by punching it, then lure it towards the hole by running there.
• Knock the bear into the water.
• Wait for the bear to slip under the ice, then stand above it.

Observed:

• The bear rears up and hits the player.  It can be seen to penetrate the ice, then fall back (Bear does not break the ice but glitches through)
• This occurs several times until the player dies or the bear eventually drowns.

Expected:

• The bear either swims to the hole and climbs out, or flails about underneath the ice until it drowns.
• The player is not damaged by its actions long as the bear is under the ice.