Coffee is a mass noun, and therefore should not take an indefinite article. But there are constructions in English that exploit and extend this distinction.
In particular, if you do use a/an with a mass noun like coffee (tea, milk, rice, tofu, beef, etc), then you are referring to a particular kind (brand, variety, version, strain) of whatever that mass noun is.
I am puzzled by conflicting opinion on a coffee.
- means that it's a kind of coffee that you're talking about, not coffee in general.
I am puzzled by conflicting opinion on coffee.
- does not refer to any kind of coffee, but rather to all coffee.
(Note that this procedure can be applied to other kinds of mass nouns, besides food and drink;
but when it does apply to food and drink, the implications are automatically about eating and drinking)