IIS 8.5 (Win 2012R2) + Automatically redirects to HTTPS when binding added

I try to understand a strange behavior I've never seen before.

I have a site. HTTP hostname-binding on port 80 are configured. The site are accessible on HTTP.

When I add a HTTPS binding (tried bout SNI and without) I can access the site over HTTPS. But, when I try to access the site over HTTP I get an 307-redirect to HTTPS.

I have no IIS redirect rules or URL Rewrite rules added. Also If I change the HTTPS port to another port eg. 44300, the redirects from HTTP (port 80) updates to redirect to the non-standard-port configured.

Worth to not, it's a .NET Core site. Is there a chance the application can check the IIS bindings and then do a proper redirect here?

Thanks for all your clever ideas!

Edit:

I find a reference to: Microsoft.AspNetCore.HttpsPolicy in a json-file. I dont have the source code.

Also, if I clear the web-root and just add a static index.htm-file the magical redirect will not happen. So, something with the ASP.NET Core application, it seems to read the port number from the IIS configuration, which sounds wired to me.