I suspect this might be a PAE problem, and not many people use PAE. There were lots of changes to x86 code that day.
I would personally start with trying to see if reverting this commit helps: http://mail-index.netbsd.org/source-changes/2017/06/14/msg085330.html I think endext=MAXIOMEM+1 may be wrong for PAE. Note some of it went into -8, so fixes need to be pulled up too.
