neant, I recommend you prepare
complete documentation of your configuration and post your question to the Project's misc@ mailing list. The number of active users who respond to OpenBSD threads on this forum is very small. I'm not aware of any of us who are using multipath routing with NAT and that is why I recommend opening a discussion with a wider audience.
Personally, I have only ever tested multipath with NAT in a laboratory environment and did not experience the results you are seeing. That could be due to the limited nature of my tests. But ... even if it were due to a component of your configuration that you did not share with us, should you share it here it is possible that no active member of this forum will be able to point you towards a root cause.
If you have never posted to misc@ before, please review http://www.openbsd.org/mail.html and http://www.openbsd.org/report.html for recommendations.