The render controller of wither had used the deprecated function over 6 years.
In the wither_boss.render_controllers.json here's a function called query.overlay_alpha which not working after 1.12.
1.21.50 Mojang fix wither's render but wither still use deprecated funtions.
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Thank you for your report!
However, this issue has been temporarily resolved as Awaiting Response
I was not able to reproduce this bug.
Is this still an issue in the latest version?
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In fact, the render controller has not changed in 1.21.50. MCPE-64537 was fixed by modifying wither_boss.animation.json
file instead.