Out of curiosity, is this problem with 
all of these 909 guns, or 
just the ones that have been tuned to have a better trigger pull?  
If the former, then yes, there's a design flaw.  If it's the latter, however, the problem may be with the tuning rather than the design of the gun.
I ask this because I know a lot of guys like to have their airguns tuned.  I never got it, myself, but neither my old Daisy or my RWS 52 have heavy triggers (unless the safety is on, then they're impossibly heavy 

).