Same thing just happened to me (doing this as a specific strengthening exercise): just now only Θ και θρόνος was correct, and I'd marked both, this time I only marked Θ και θρόνος, and guess what, I should also have marked Θήτα και θρόνος. Grrr.
Oh, I see you said "strengthen". Those have a problem not accepting all translations for some reason. Sorry for the inconvenience... Both are added in the incubator. I think those "strengthen" problems will be solved after beta.