Should this not be "quintus miles" if we are speaking of one soldier?

FYI: We've already updated "milites" to "miles," but it often takes quite a few days before the update goes through the system and is reflected to users.

Milites is a plural and does not match with quintus. It should be miles.

Latin in exercise sentence reads "quintus milites"

Miles?

Yes, 'milites' should 'miles' if we're taking about only one soldier.