Subject explains everything. RSS feed of "FreeBSD Security Advisories and Errata notices" (https://www.freebsd.org/security/feed.xml) doesn't include information about Security Advisories at all.
BTW, There have been "SECURITY ADVISORIES" and "ERRATA NOTICES" sections in the top page of FreeBSD web site and both of them have been links to RSS feed. Before the migration to Hugo/AsciiDoctor they pointed to different RSS feeds. But after the migration both of them point to above RSS feed. Is it intended change?
I'm having the same issue. The weird thing is if I open it in Thunderbird I see both Security and Errata however if I open it in a browser-based RSS reader I only get Errata.
OK, by downloading raw feed data I understand what is happening. In feed data entry for EN is listed in decremental date order at first, and next entry for SA is listed in same order. I use the feed date with my custom program and it uses the data as is. So it looks as if only information of EN was included. I don't use Thunderbird but probably it sorts all entries in decremental date order before using it. So user see entries of both EN and SA.
So in conclusion I'm not sure if this is a bug. But personally I'm happy if feed data is changed so all entries are listed simply in decremental date order.