The current implementation of the JsonConverter does not allow the use of Ulid as a dictionary key, resulting in an exception: The type 'System.Ulid' is not a supported dictionary key using converter of type 'Cysharp.Serialization.Json.UlidJsonConverter'. The solution is to override WriteAsPropertyName and ReadAsPropertyName. Here is a no-nonsense extension of your converter class, but would be much better not having such an extension and just relying on yours.

public sealed class MyUlidJsonConverter : UlidJsonConverter
{
	public override void WriteAsPropertyName(
		Utf8JsonWriter writer,
		Ulid ulid,
		JsonSerializerOptions options) => writer.WritePropertyName(ulid.ToString());

	public override Ulid ReadAsPropertyName(
		ref Utf8JsonReader reader, 
		Type typeToConvert, 
		JsonSerializerOptions options) => Ulid.Parse(reader.GetString());
}