Have you tried the so called Simpson map?
I think that one may have been done for a 900RR converion.
Have you tried the 2 Bros Full System map from the P C list of maps? (M112-004)
All 4:1s give a flat spot somewhere, it's just a matter of how bad and where. Honda would push the RPM location down for a 900 RR, but it's likely in the useable zone of a 919. If the fueling is off, the condition is aggravated. You may be sensing this as well, in terms of the overall map across the revs that is.