Why is GetClientCertificateAsync method always null?

Question

I'm making a httpclient request with a attached certificate to the handler. We're migrating to .net core from .net framework, and the older code worked, but now the problem is that we can't get the value from clientcertificate (https://docs.microsoft.com/en-us/dotnet/api/system.net.httplistenerrequest.getclientcertificateasync?view=net-6.0).

But we have a client react app that logs in and from the browser we can successfully send in a certificate, but in code (when making a httpclient request) it doesn't work.

There seems to be a issue (https://github.com/dotnet/aspnetcore/issues/28557), but It did not resolve my issue.

https://docs.microsoft.com/sv-se/aspnet/core/security/authentication/certauth?view=aspnetcore-5.0#kestrel

https://docs.microsoft.com/en-us/dotnet/api/system.net.httplistenerrequest.getclientcertificateasync?view=net-6.0

Anybody who has a clue about solutions to this problem?