Does OAuth Technical Profile send request to token endpoint without accept header (TrustFramework)?

Hello,
does OAuth Technical Profile send request to token endpoint without accept header (TrustFramework)?
There is this local identity provider I want to connect to. I already have Google and other built-in providers up and running but I have a problem when I want to implement something that is not. The problem is that the call from AAD to /token endpoint ends in failure (406 - not acceptable) - seen from a trace in AppInsighs, all I see is ContentType: Json, but nothing about accept header.
I tried to replicate this behaviour in postman and it seems to be the case - 406 is returned when no accept header is present in the request.
Now I am not sure if that is a problem on AAD side or the provider (that they require accept header).
And one last question - is it possible to see the requests of OAuth Technical Profile endpoints (TrustFramework)? The only reason I am writing here is that I am unable to find anything about such issue and I can't even check how the requests are sent to the providers.
Thanks in advance