Una corda: upright lost motion compensator?

[Cy Shuster]

I have an S&S model F console (sympathy accepted, but it's a family piano). 
I regulate uprights so that the bridle straps don't move the keys down when 
the una corda is depressed.  But on this piano, all but one key moves down 
with the una corda, and this nicely removes the otherwise excessive lost 
motion.

Bug or feature?

--Cy--
ABQ, NM