mock function is executing contract logic despite mocked instructions

[aazhou1]

Describe the bug
A stubbed function of a mock contract is reverting on something in the contract logic. This doesn't make sense to me since it shouldn't care about the contract logic if stubbed. When running the unit test, we get the error Error: Transaction reverted: function returned an unexpected amount of data. The line corresponding to the error is purchaseTokenInstance.decimals() where purchaseTokenInstance is a IERC20MetadataUpgradable. If I use Hardhat console logging, the function doesn't actually execute and spit out values.
Reproduction steps

contract.function.returns(BigNumber.from("1000"));

Expected behavior
calls contract.function should just return ("1000") in unit tests, instead of reverting as in the description.

[wei3erHase]

ey @aazhou1 , could you provide an isolated example that replicates the bug?