Answering my own question in case someone else has the same problem in the future:

In the code provided, even though we extend the IUserStore to IEmailUserStore when mocking, we still need to manually set the boolean value that enables email support for DefaultUI in _userManagerMock before passing it as a parameter to _signInManager:

_ = _userManagerMock.Setup(x => x.SupportsUserEmail)
    .Returns(true);