Unexpected error "feature 'ref struct interfaces' is not available"

[jjonescz]

Version Used: 4.13.0-3.24612.1 (c30de30)

Steps to Reproduce:

Compile with LangVersion=12:

using System.Diagnostics.CodeAnalysis;
interface I
{
    void M();
}
struct S : I
{
    [UnscopedRef] void I.M() { } // errors here
}

Expected Behavior: Only error CS9102: UnscopedRefAttribute cannot be applied to an interface implementation because implemented member 'I.M()' doesn't have this attribute.

Actual Behavior: Additional error CS9202: Feature 'ref struct interfaces' is not available in C# 12.0. Please use language version 13.0 or greater.

But there is no ref struct implementing an interface.