Just to be clear we are talking about bitterness and not something else here’s a breakdown of what sometimes get confused-
Bitterness: a flavor primarily caused by phenols and ethanol. Think citrus rinds and cocoa powder.
Astringency: a dry feeling that can effect the whole mouth caused by phenols, namely Tannins. Think strong black tea or chewing on an aspirin.
Tart: a flavor associated with certain acids, in apples mostly malic. It contributes to the “dryness” of the beverage and some people describe it as sour.
There are very few compounds produced during fermentation that would yield a bitter flavor other than alcohols. A variety of compounds can be masked by the presences of sugar. As fermentation proceeds, and more and more sugar is eaten up, those compounds become more available to produce perceptions.
A few answers that would be helpful:
1. Where did the juice come from?
2. Are you using bittersweet or bittersharp apples?