I think I figured something important out here; a clue. After studying the .fac report some more, I realized that the reason I had chosen GBPCHF was because of it's excellent OptF component rating on GBPCHF:S (short).

In looking more closely at this report (which I've sorted descending based on best OptF to worst)... here are the other GBPCHF OptF results:
GBPCHF:S 0.808 (short)
GBPCHF 0.141 (combined long and short)
GBPCHF:L 0 (long)

It showed up on my radar as one of the top 7 assets while prospecting, which was primarily due to the short component (only). Typically when I see a good component, I assume it means both shorts/longs will also be good. But apparently not in this case.

If I simply disable long trades on this logic altogether, I get the reasonable result I'm expecting. It never occurred to me that I might want to do this LOL. In looking at the chart of GBPCHF, it makes sense to me because long term (since 2008) it has been in a steady downtrend. If I was trading discretionary, I might only take shorts on this pair.

It's easy enough to add a switch for assets to restrict them to only-long or only-short. I think I can even do it automatically, since I take an OptF measurement every step of the process (out of laziness to turn it off). I will give that a go and see if I can make more progress on this pair.