Water issues sound like a not unreasonable shout initially but what doesn't add up is
@NeipaNutter is saying the beer prior to packaging tastes good but after carbonating develops the problem.
Astringency isn't a bad shout either but, again, why would you not notice it prior to packaging? Unless the carbonation was somehow amplifying it. Worth noting that astringency is not a taste, it's a sensation. A dry puckering sensation - like the kind you get from dry red wines or over brewed tea (where it comes from tannins).
My money is on oxidation/beer staling. Just because it doesn't look oxidised doesn't mean it isn't. I find that oxidation in pale hoppy beers can often come through as a slightly harsh sweetness. The kind you get from hard sweeties that catches at the back of your tongue.
Overall though, I agree with what
@Ghillie says. If you can describe it accurately, and due to current circumstances cant really share it with anyone, it'll be very hard to pinpoint.