From the comments and doing a bit more research on PostMan's development.
The simple extension version does not currently default the Content-Type of the request based on the dropdown in associated to the raw post body header. You need to instead the Content-Type explicitly using a header explicitly.
Looking on Postman's Github, it seems this is not a new bug. Digging through issues, it seems that there is also a Packaged App version of Postman which behaves properly, and gives you distinct options for XML (application/xml)
and XML (text/xml)
even.