YamlDotNet: Exception during deserialization of nullable enum >= YamlDotNet v9.1.0

Problem deserializing nullable enum with YamlDotNet v9.1.0, no issue on v8.1.2.

Is this intended?

public enum LanguageMode
{
    FullLanguage = 0,

    ConstrainedLanguage = 1
}

public sealed class ExecutionOption : IEquatable<ExecutionOption>
{
    [DefaultValue(null)]
    public LanguageMode? LanguageMode { get; set; }
}