How/when do custom JsonConverter instances get added? (.NET6 in-proc)

[thomas-brown-rs]

We are running into an occasional problem where we get a null reference exception from JsonSerializer’s GetMatchingConverter method; after some investigation this appears to be related to the converter collection being thread unsafe. Is anyone aware of how converters are added if IMessageSerializerSettingsFactory is not overriden as per https://learn.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-serialization-and-persistence?tabs=csharp-inproc#customizing-serialization-with-dependency-injection? We are hoping that overriding that implementation to our own provider will fix this issue but just looking for more information.

We are using a .NET 6 in-proc function app with functions V4 and the latest version of the durable tasks package.