In the second stage of fighting the Wither boss (when It's at Half health points) some moving Lines appear around it (without ray tracing), which is a part of It's visual, but this doesn't function with RT turned on.

The wither at half HP:

in this stage the lines have an animation/they move while with ray tracing It all becomes a blocky mess.
The Wither at Half HP with Ray tracing:
 

Steps to Re-produce:

1. Enter a Ray tracing enabled world or activate a pbr resource pack in your own world
2. Summon a wither boss
3. Take it to half HP
Observed results: Wither texture is broken with ray tracing on.
Expected results: It should look similar or better to how it is with ray tracing turned off & animations should be present.

additional info: this only affects second stage of the wither, before half hp