Good question, and I think you are right. My style guide says: At the beginning of a sentence, spell out all numbers, even though similar or coordinate numbers are written as numerals elsewhere in the same sentence.Is it permissible to begin a sentence with a numerical number?
Lo dice el Diccionario panhispánico de dudas.OK.
¿Quíen dice que no lo hace?
¿Para qué se pone?