Why does API Gateway remap request headers

I've come a across a behavior that I don't succeed to explain on the API gateway service.

When running a request on a type HTTP api gateway with the headers Content-Type: application/x-www-form-urlencoded and Accept:application/json

The request header sent to the back-end server is then changed to Accept=application/x-www-form-urlencoded without any configuration on my side to change It. I did not find any documentation describing this behavior. When using the integrated test tool in api gateway the behavior is not the same and the headers stay as requested.

Here is the logs on the integrated test tool

Method request headers: {Accept=application/json,...,Content-Type= application/x-www-form-urlencoded}
...
# Accept header is correctly forwarded
Endpoint request headers: {..., Accept=application/json, ...}
...
# The request response has the correct Content-Type
Method response headers: {..., Content-Type=application/json}

When looking at the gateway execution logs a different behavior is observed:

Method request headers: {Accept=application/json, ..., Content-Type=application/x-www-form-urlencoded}
...
# The request header get overridden by some API gateway behavior
Endpoint request headers: {..., Accept=application/x-www-form-urlencoded, ...}
...
# The server reject the response and return a 406 with a Content-Type=application/json
Method response headers: {..., Content-Type=application/json}

Looking at the difference between the integrated test tool and the feature make me think about this behavior being a bug, but I may have missed something