Hello @Gert Vloo
I have run this by the APIM team and they have confirmed that this is not a bug but an expected behavior that honors the RFC2616 spec for the header value as *TEXT, which is an 8-bit sequence of data excluding control characters: https://datatracker.ietf.org/doc/html/rfc2616#page-16
I hope this brings clarity and helps you proceed with your project.