Maybe the question was not as well made as I first thought, but two of the constraints in the OP were
constant fuel flow and
constant RPM. Therefore I would suggest that pumping losses - changed though they may be - do not explain the need to adjust MAP
whilst maintaining constant fuel flow.
Lycoming advise this on power setting:
"to maintain constant power, correct manifold pressure by approx 0.18"Hg for every 10F variation in induction air temperature from standard altitude temperature. Subtract manifold pressure for temperature below standard."
That sounds like a density correction to me.