How to allow all https regardless of validity in .NET Core HttpClient?

I found the solution:

var httpClientHandler = new HttpClientHandler();

httpClientHandler.ServerCertificateCustomValidationCallback = (message, cert, chain, errors) => true; // DEBUGGING ONLY

var httpClient = new HttpClient(httpClientHandler);

Although it is still unclear to me why the certificate is not considered valid after running dotnet dev-certs https --trust.

Use the sample below from here

var httpClientHandler = new HttpClientHandler();
// Return `true` to allow certificates that are untrusted/invalid
httpClientHandler.ServerCertificateCustomValidationCallback = 
    HttpClientHandler.DangerousAcceptAnyServerCertificateValidator;
var httpClient = new HttpClient(httpClientHandler);