For those who stumble onto this question from a search engine (like me), here is how I fixed this. .Net Core changed how ServicePointManager works, so instead of doing the override suggested in the Splunk docs, you need to provide a custom handler to the Context object. Like so:
var handler = new WinHttpHandler();
handler.ServerCertificateValidationCallback += (sender, certificate, chain, sslPolicyErrors) => true;
var ctx = new Context(Scheme.Https, "your_splunk_enterprise_url", 8089, default, handler);
this._splunkService = new Service(ctx);
await this._splunkService.LogOnAsync("username", "password");
After doing this the LogOnAsync method no longer returns the error about invalid certificate validation procedure etc. My app is targeting .NET Core 3.1.
Mods - can we get this added to the official documentation, perhaps?