I recently implemented multiwan successfully on a clean install of Opnsense.
DNS over multiwan failed to function as anticipated based on the existing how-to: https://docs.opnsense.org/manual/how-tos/multiwan.html
Failover was successful in all aspects, but DNS was non functional on the failover gateway. I identified the issue as not having enabled "allow default gateway switching" in System : Settings : General
I believe this constitutes a required step to get multiwan functioning and should be added to the documentation.
I'm happy to submit this as a change to the opnsense docs via a PR but prior to doing so felt it was important to check in here on correct procedure for submitting a doc update for this project. Should I field this in the general forums first? Should I draft an addition to the how-to and PR?