One of the options when you hover over "maïs" is given as "sweetcorn", "the sweetcorn is black" is marked as incorrect, though.
This is a problem that I have noticed every time "maïs" is said or written: French say "LE maïs" not "LES". If it were "les maïs" you would have the verb and the adjective in agreement, ie in plural "les maïs sont noirs"
Yes, this is why when the voice says "LES maïs", you should expect the verb and the adjective in agreement. Which is not the case here, she systematically says "les" instead of "le".
Could they have fixed this already? I'm certain that i'm hearing "LE maïs est..."
Edit: Either that, or we're listening to different versions
We don't refer to corn as a plural in english either. We don't say 'the corn are ready', we say 'the corn IS ready'. Nor do we refer to other words as plurals. We don't say 'i'm getting my hairs cut'. We use that as a singular as well.