Possibly the difference between the usual British English 'have you...?' and the American English 'did you...?'

True it is the difference between British English (probably the people who developed this) and American English. After being corrected I can see that "have you washed" makes sense, but it is not what comes to mind when translating - especially when I'm doing a timed practice.
Most Americans just don't use that type of phrasing. I've requested that it should be marked as correct.

eto is commonly used to mean both 'again' and 'yet'.

ymolchi = 'to wash oneself' or 'to get washed'

So the most straightforward translation is 'Have you washed yet?'