Wither is meant to deal damage and make hearts harder to see, and this behaviour makes poison able to nullify half of that seemingly unintentionally.
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Can confirm, and it works with freezing too.
But unless you expect them to make different heart textures for each combination of poison, wither, and freezing, I don't think they would be able to do that.
Alternatively, a more realistic solution, make it cycle between the different heart states if you have multiple heart effects.
Here are some commands to help reproduce this issue:
/effect give @s regeneration 60 7
/effect give @s posion 60
/effect give @s wither 60
/effect clear @s
This is related to MC-205102.