This was a click-the-correct-word question with seòmar and seòmraichean available. I was not sure if seòmraichean would fit. Is it 'S e or 'S iad? Even if the latter, am I correct in thinking that you will quite often hear the former, regardless of number or gender?
Would it make a difference what the question was? Perhaps if the question was What are they? the answer would be 'S iad, but it the question were What is the problem? you would say 'S e? (There was no English displayed in the question.)
Did you report it? I think (but am not sure, definitely don’t take my word for it) that ’s e is today also common for plurals. I believe Duolingo randomly chooses wrong answers from some set of similar words/sentences and the contributors must manually ban those that are potentially grammatically correct but not expected, so they might have missed it.
I believe the official teaching is that you should use 's e, 's i or 's iad as appropriate, but that universal 's e is quite common.
To my logic, you would have to use 's e if the question did not imply number or gender, similar to French c'est, which sounds suspiciously similar, means exactly the same thing and comes from a country with a Celtic past. Just saying.
Interesting that they can ban specific wrong answers. I will report it next time if I didn't this time. It just seems an odd premise that Duolingo works on that words similar to the intended answer are assumed to be wrong.