This is not a bug... Its just the cooldown for the shrieker, and after some time it will give the darkness effect again...but from what i've noticed, the cooldown seems to be a lot longet in bedrock than java... So in the end, its just a parity issue.
This is a feature...not a bug
Does this issue happens only in the betas?
Or does it affect previous versions aswell?
Yes it does...for some reason bedrock can't handle too many resource packs at a time, and if you have a world with too many addons it might corrupt the world
The Wither is a undead mob...so potions of harming do heal it...
And If you do want to kill it with potions, try an instant health potion.
And the reason for the Wither being in your realm in the first place was because someone spawned him.... There are a lot of tutorials on youtube showing how to spawn a Wither
This is a feature...it was introduced in the update aquatic, and its present in java edition too...
Do you have any resourse or behavior packs? If so...you have to delete them..
I had this issue before, and deleting those packs helped with performance by a LOT...so i recommend you doing that...
Just in case remember to backup your worlds if you do have any packs installed
This happens to me every time i join a server for example: lifeboat, mineville and cubecraft...
This happens in more than one platform (i tested on my xbox one and phone) and i had this issue on both devices...
Still affects version 1.18.30.23 preview