Demo project:
project.zip

Environment:
.NET Core 2.1 on Windows 10 and Debian 9

Steps to reproduce:
Download the demo project and run it on Windows and Linux.
It will print "Cert valid? True" on Windows, and "Cert valid? False" on Linux with exception.

About the demo:
A HttpClient with ServerCertificateCustomValidationCallback, Set ChainPolicy.RevocationMode = X509RevocationMode.Online in callback