- Why do I get an html form as a response when sending the consent form to the backend ?
You are probably using fetch or such. You must provide an actual form, as done by the html component.
If you don't do so, you will fail the backchannel and weird behavior is going to happen.
- I want a custom behavior on the server and the package already provides behavior that I cannot override and / or have unwanted side effect.
You can use the package CK.DB.OIddict instead of CK.DB.AspNet.OIddict.