“A transformation ability . . .? It must be pretty impressive to turn even his presence completely human.”
Not only Hansa but most Executors could see through transformations and disguises that relied on ordinary magecraft or hematophage peculiarities.
But the sight of that transformation, which was almost like the creature had swapped out its very soul, reconfirmed for Hansa that Jester was an “enemy” he could not afford to underestimate.

Fate/Strange Fake: Volume 6, Chapter 18 ("As Dream and Reality are Both Illusion I")