Power Automate Custom connector Authorization header

Question

Hello,

I have to connect to an external GraphQl API. I created a Custome Connector set up everything for the body and the url. When I go to testing and create the connection which takes a JWT as API Key. I input all the fields and test connection. Then I always get a 500 Internal Server error as a result. When I check the response that is send it looks fine with exeption to the JWT in the Authorization header. It is JWT that seems to be related to Azure because those are the audiance and issuer fields:

"aud": "https://apihub.azure.com",
"iss": "https://sts.windows.net/afc7f9ae-5fec-4b50-b635-086033768b4b/",

So my question is can someone please explaine why the Authorization header is replaced with this Azure JWT?

I even tried a policy to override the header value it still does not work. My current assumption is that this wrong JWT is send to the GraphQL API which answers with a 500 in case of wrong authorization header. I tested that with Postman.
I am thankful for any lead in the right direction. Because I need it to work for my client.