Coffee is a rare drink in that it needs to be fresh to taste good. Even the slightest few hours it stays and sits it can be unsatisfying. So, the easy answer to this question is yes coffee will become bitter if you leave it out too long. But, the coffee maker itself can’t be blamed for this as even refrigerating it and re-warming it back up will not leave the same taste. Most coffee makers, specifically most of the models on this list, are designed with some sort of inside insulation to keep the coffee hot for an extended period. Do not confuse hotness with freshness, though, as there is nothing the coffee maker can do to achieve this.
With this being said, though, there is nothing wrong with drinking coffee after it has lost its flavor. But, it resembles drinking flat pop in which both the flavor and texture are completely altered. When making more coffee than you need, try not to let it sit for several hours.