I read "There's a rule that zeros must never be rounded!" as either
CustomMath.Round(x, y, 0) == CustomMath.Round(x, y, 1)
or
CustomMath.Round(x.0, 0, 0) == x but CustomMath.Round(x.01, 0, 0) == x + 1
So I took zeros to refer to a zero as a rounding decimal digit not as the entire number.
It took some trial and error with the tests to work out what it was intended to mean.

Not an issue?

I appreciate your nice feedback.

Actually in the description is stated that zeroes must not be rounded as follows:
"There's a rule that zeros must never be rounded!"

I've removed the JS.

Why

Assert.AreEqual(0, CustomMath.Round(0.0m, 0, 0));

while

Assert.AreEqual(2, CustomMath.Round(1.0m, 0, 0));

It's inconsistent. If you insist to do this, please put it in the description.

• You forgot to remove the dummy JS version.
• Needs Random Tests.