Summary:

First of all, I don't know if this is considered a bug, but Wither Boss when summoned in Minecraft Bedrock is very different compared to Minecraft Java

Steps to reproduce:

1- Get 4 soul sands and 4 Wither skeletons heads.

2- Place the soul sands in a «T» shape with two vertical blocks and he rest on each side of the last vertical block. Thus making three horizontal blocks.

3- Place the three heads of Wither Skeletons on top of the three horizontal blocks and the Wither Boss will be summoned.

Observed results:

The Wither Boss when summoned in Minecraft Bedrock, he turns all white and sometimes he moves in a very strange way from side to side while his health bar fills up.

Expected results:

He should be immobile and with the normal color of the Wither Boss, but with a bluish and transparent shield while his health bar fills up. Same as Minecraft Java.
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I left some videos so you can see the difference. Please, if you can, make the Wither Boss summon in Minecraft Bedrock equal to Minecraft Java.