Madam answered my question.
"Vampires move and see better in the dark, you see. That girl has good eyesight, even among us. So she probably could judge at a glance that even when wearing maid clothes, you had the body of a boy instead of a girl.
The head butler Merry bowed his head and apologized.
"I checked the documents again, and it was certainly written ‘male’ in the gender column. It was my fault for not confirming baa~."
"Hahahahahaha! It's all right, it's all right! It was my fault for immediately buying him at the shop without checking in the first place!"
"...... I could tell from the smell from the start."
"Eeeeh!?