So this issue has already been noticed here:
https://github.com/joomla/joomla-cms/issues/39332
My component gets accessed via the API by a big company who is using the same access with hundreds of partners all over the world.
What they do is sending only this as header:
Cache-Control: no-cache
Content-Type: application/json
It works with all partners but Joomlas route function is missing the $mediaType and throwing a:
"Could not match accept header"
What can be done to have a successful API access without the partner having to send a accept header?