I did see that. Unfortunately I don't have any experience with pf and especially not with synproxy. I also couldn't find anything about synproxy and mac os on the interwebs; not even like stackexchange posts about it not working with no solutions (like i saw for pf and divert). That said... I did see this post about it not working on NetBSD in a similar way. The described symptoms sound similar to your experienced symptoms, so I wonder if their workaround will work, too. They've suggested adding a second loop back interface with a new RFC1918 address, but I don't see why you couldn't just redirect to 127.0.0.1, or add a RFC1918 alias to lo0. So your rule might be
pass in quick proto tcp from any to X.98 port { 80 443 } flags S/SA rdr-to 127.0.0.1 synproxy state
Or whatever other IP you add. If you're doing IP based virtual hosts, you'd need to tweak your apache config a smidge to manage, but not too bad.