When the illusioner fires an arrow, it shows the arrow being loaded in the bow, but it is then fired with the feathers of the arrow starting from behind the head. This only happens in the non-illusion state.
To duplicate, have the illusioner attack a mob and view from the side.
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I wouldn't say it's unsupported since some illusioner tickets are valid, unresolved and have Mojang Priority assigned.
Duplicate of MC-48071, the illusioner is an unsupported mob.