.NET Core 2.1 has a new class EnumTypeModelBinderProvider. FlagsEnumModelBinderProvider must be inserted before it. Otherwise, the Enum types do not reach the GetBinder method.
Thank you for your feedback and this problem is fixed in the version 1.3.8. I'm closing this issue and please feel free to reopen it if you have any new question.
.NET Core 2.1 has a new class EnumTypeModelBinderProvider. FlagsEnumModelBinderProvider must be inserted before it. Otherwise, the Enum types do not reach the GetBinder method.