OK problem solved. Thinkingcap suggested I turn off the low cut in the master bus EQ because, “From phase shifts or resonances, that can be caused by filters. Switch it off and surely everything is as it should be (or switch the Limiter last, instead of the EQ…”
So I turned off the EQ and everything read correctly, though I think it’s odd that pulling out 1db of low end would cause a spike. Anyway, I’m happy now, and hopefully this will help anyone with the same problem.