Thanks for the welcomes!
This is a Witeside UD5160 1+1 1/2"x1/2" Carbide Compression Spiral Router Bit bought new off eBay.
I can't reliably get calipers on it because of the design. There's no play. I actually took out out, replaced it and cut a new hole.
It appears that the ridge is created at the transition between the first spiral and the second--as though that first spiral section is cutting 1/32 too small.
Is that feasible? I tend to not like to blame my tools, but the fact that all my mortises transition to a smaller width at exactly the same place is a bit suspicious.
Also, I tried a Freud 1/4" spiral bit (different collet, but otherwise same setup) and it worked fine.
I can clean up the problem with a chisel, but man, I don't want to...