".AspNetCore.Correlation." state property not found. Azure B2C

Question

".AspNetCore.Correlation." state property not found

We have been getting a number of these errors in our Web App whilst hitting the standard /signin-oidc callback url. I am using .net core 2.1 and using the .AddAzureAdB2C(... from the Azure samples.

Nothing crazy in our setup, afaik. We are leveraging 3 of the the OpenIdConnectEvents

1. OnRedirectToIdentityProvider to add a Guid to the state parameter -since the iOS12 change to drop the auth cookie to SameSite.None and also to pass a GA tracking id through when it is present from the referring site.
2. OnRemoteFailure to redirect to appropriate error pages.
3. OnTokenValidated to validate the Guid added in 1. and strip the GA id out again.

Our setup is a load balanced (traffic manager) azure environment, with 2 app services (north/west) - both sharing the same application ClientId and ClientSecret, RedirectUri etc. The DataProtection key(s) for the App Services are shared and held in Azure Blob storage.

Can't really see what the problems is, done a fair bit of searching around but getting nowhere. If anyone can shed any light, it would be much appreciated.