The includes (formerly called contains and include) method compares objects by reference (or more precisely, with ===). Because the two object literals of {"b": 2} in your example represent different instances, they are not equal. Notice:
({"b": 2} === {"b": 2})
> false
However, this will work because there is only one instance of {"b": 2}:
var a = {"a": 1}, b = {"b": 2};
_.includes([a, b], b);
> true
On the other hand, the where(deprecated in v4) and find methods compare objects by their properties, so they don't require reference equality. As an alternative to includes, you might want to try some (also aliased as any):
_.some([{"a": 1}, {"b": 2}], {"b": 2})
> true