truepolymorph:
  icon: snowball
  actions:
    cast:
      - class: ModifyEntity
  parameters:
    target: other
    range: 32
    delay: 5000
    type: husk

Using modify entity to change the mob type causes the entity to lose all magic properties

This will be fixed in the next dev build.

This also fixes a similar issue with Grow and Shrink which has been a long-standing request.

Thanks for the report!