It's because Firefox uses its own dns resolver when it uses DoH, but if DoH is disabled it just calls getaddrinfo from libc to look up a domain. And unfortunately getaddrinfo is a very minimal API, that doesn't have any way to resolve arbitrary record types like HTTPS or SVCB, it just returns ip addresses. There is a bug[1] for supporting HTTPS records when using the system dns, but I haven't seen much progress on it. And sadly, it probably requires different code for each OS.

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1852752