You answered this question months ago but I finally got round to testing it and you were right. Had to file the braze on slot a bit to get the derailleur lower, but since I did the chain hasn't dropped once. Thanks!
I am always surprised how many bikes I build that shift well in the stand and not on the road.
I tend to ride them all for a couple of miles to let it all settle and re-adjust.
FD poorly adjusted?
Either set up too high or goes back too far in.
Re-check and fit a chain catched so it can't physically drop