Json serialization error when saving block with enum property and integer backing type

Hi

We have recently upgraded from CMS 11 to 12.

We're getting serialization error when trying to save a block containing an enum.

The backing type is integer.

The easy fix is adding preferredUiJsonSerializerType="Newtonsoft"to module.config.

But the question is: how would a fix this using the Core built-in serializer? I have tried adding a converter in JsonOptions, as a converter and as a converter factory, as well as a service. But the converter is never called? Obviously without the above change to module.config, or set to "Net".