I answered A hairy dog, and got told I had a typo and the correct answer was An hairy dog
I was taught (some forty-five to fifty years ago) that you only put an if the 'h' at the beginning of the next word is not pronounced, so it's An hour and An honest man but A hotel and A hairy dog
'an...' had crept into the system as an alternative somehow. Now deleted!
As EllisV says, though, 'a...' should not have been rejected.
"A hairy dog" is in the system and is the prefered answer, so this probably a glitch.
Computers are like that. Sometimes they do the wrong thing just for the entertainment value. I should know, I work in IT Support.