I'm assuming you are referring to a Listening Exercise.
Let us go back 3 years. At that time we disabled this sentence and it was no longer used for Listening Exercises...as evidenced in the lack of new comments...is there another comment in the last 3 years. NOt until today. So, saying "still being rejected!" is incorrect
However, Duolingo has recently changed the Audio System, and we now need to disable each sentence individually to assure that homophones are not used for Listening Exercises. I have just done that and we will continue to do that for each and every sentence. But for 3 years it was not used for Listening.
If you have read my comment above you will see that I disabled this sentence so it will not be used as a Listening exercise. Perhaps the program doesn't change in a few hours. I have an idea. If you get this exercise again...don't answer...press "Cannot do this exercise at the moment." or give the plural answer which you've discovered is what the system expects as you now know. It will not change to accept the singular it will just disappear and never be used again that's what diable means.
In any case, I've done what needed to be done to relieve learners of this exercise.
BTW isn't this the Plural Skill...so it's not really surprising that the plural would be expected. But as I said above...more than once, since the singular and the plural sound the same this exercise should not be used at all.