If that so, you can disable the Cloudflare cache for your /feed/ URL using Page Rules.
Do you already use some of the Page Rules, if any?
example.com/*feed* should prevent these pages from being cached.
Or disable the Brotli compression for the needed URL:
Moreover, I have not yet seen that kind of an issue using RSS feed from WordPress and having Cloudflare with “Cache Everything”.
Have you tried validating it via Direct Input?:
Does your Web server return as content-type MIME
gzip - regarding the Cloudflare?
Or is there any optimization at the origin/host of the content?
I got returned
content-type: application/rss+xml; charset=UTF-8