This might be because the EM has gone bad - in which case chuck it away.
Or it might be something to do with how sweet it is.
Sweet and sour are very closely linked. If you've ever had no sugar fizzy drinks that are sweetened with nutra sweet or similar you'll notice that almost always you get a bitter aftertaste after the initial sweet hit from the sweetner.
This turns out basically to be because they are too sweet.
If you put a sugar sweetened substance onto your tongue , then the sugar molecules fire the sweet sensing bits of your tongue, which are right next to the bitter sensing bits of your tongue. Most artificial sweetners 'over excite' the sweet sensing parts of your tongue, which, as they are right next to the bitter sensing parts of your tongue activate those as well. Hence artificial sweetners are plagued with a bitter aftertaste.
(If you can come up with an artificial calorie free sweeetener that doesn't do this you will make BILLIONS btw)
Ethyl Maltol isn't a sweetner is more a kind of sweet version of salt. It develops and accentuates already existing flavours. Try a bag of chips with no salt on - than add some salt, the difference is really big.
EM does the same kind of thing to food tho it has a slightly sweet taste and not a salty taste.
It could be that your mix is too strong and is overexciting the sweet tastebuds which then leads to bitter.
Or it might just be a duff bottle somehow.