Significant changes in response when flow and gradient are changed

I am trying to optimize a method for narcotics to include buprenorphine and norbuprenorphine.  Why is there a significant change in response (AUC) for these two analytes when the flow and gradient are changed?  None of the other 11 analytes perform this way.  The same mobile phases and instrument conditions (other than flow and gradient) are being used and baseline resolution is achieved in both methods.  I'm using a 1260 Infinity LC and 6420 MS/MS.

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