I suppose that the issue faced is caused by the fact that in .Net Core 3.0 the Newtonsoft.Json package is no longer included by default. Instead, the application would use the System.Text.Json assembly. That would also lead to the fact that all field names will be translated from PascalCase to camelCase when sent from server to the client. In order to avoid that, you should explicitly turn off that serialization option:
.AddMvc(options => options.EnableEndpointRouting = false)
.AddJsonOptions(options => options.JsonSerializerOptions.PropertyNamingPolicy = null);
If the above does not help, I would like to ask you to send us the sample template solution, where you have observed the issue in question. This way we will be able to troubleshoot the problem locally and to provide you with the most appropriate assistance on that case.
I hope that this helps.
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