Nullability mismatch in parameter of explicit interface implementation is not detected

[AlekseyTs]

        [Fact]
        public void ImplementingNullableWithNonNull_Parameter()
        {
            var source = @"
interface IA
{
" + NonNullTypesOff() + @"
    void M1(string?[] x);
" + NonNullTypesOff() + @"
    void M2<T>(T?[] x) where T : class;
}
" + NonNullTypesOn() + @"
class B : IA
{
    void IA.M1(string[] x) => throw null;
    void IA.M2<S>(S[] x)
        => throw null;
}
";
            var compilation = CreateCompilation(new[] { source }, options: WithNonNullTypesTrue());
            compilation.VerifyDiagnostics(
                // (5,19): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' context.
                //     void M1(string?[] x);
                Diagnostic(ErrorCode.WRN_MissingNonNullTypesContextForAnnotation, "?").WithLocation(5, 19),
                // (7,16): error CS8627: A nullable type parameter must be known to be a value type or non-nullable reference type. Consider adding a 'class', 'struct', or type constraint.
                //     void M2<T>(T?[] x) where T : class;
                Diagnostic(ErrorCode.ERR_NullableUnconstrainedTypeParameter, "T?").WithLocation(7, 16),
                // (7,17): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' context.
                //     void M2<T>(T?[] x) where T : class;
                Diagnostic(ErrorCode.WRN_MissingNonNullTypesContextForAnnotation, "?").WithLocation(7, 17),
                // (12,13): warning CS8617: Nullability of reference types in type of parameter 'x' doesn't match implemented member 'void IA.M1(string?[] x)'.
                //     void IA.M1(string[] x) => throw null;
                Diagnostic(ErrorCode.WRN_NullabilityMismatchInParameterTypeOnExplicitImplementation, "M1").WithArguments("x", "void IA.M1(string?[] x)").WithLocation(12, 13)
                );
        }

Observed:
No WRN_NullabilityMismatchInParameterTypeOnExplicitImplementation warning for B.IA.M2. Reported when mismatch is in return type, see existing unit-test ImplementingNullableWithNonNull.

[gafter]

I don't see why either warning is deserved. In general, contravariant changes in an argument's nullability, and covariant changes in the return type nullability are safe and do not deserve a warning.

[gafter]

Oh, I got that backwards! Both deserve a warning in this test.