You simply shouldn't have any challenges whatsoever on an RSS feed. They're literally meant to be read by a machine.
The issue here is that Cloudflare's content type check is naive. And the fact that CF is checking the content-type header directly needs to be made more explicit OR they need to do a file type check.
There were compatibility issues with other type headers, at least in the past.
'application/rss+xml' seems to be the best option though in my opinion. The '+xml' in the media type tells (good) parsers to fall back to using an XML parser if they don't understand the 'rss' part, but the 'rss' part provides more accurate information on the content's type for parsers that do understand RSS.
All that said, it's a mess.