A level change in any existing presets is very common in major amp model updates. It is almost never a bug.
Also, Cliff reworked some of the MV tapers part way through the betas.
I think the point is that the different Mark IV models are all very similar with some minor mode switches, and all used to be at the same level. Now, one is louder. He was reporting it as an indication that something in one might have been typod to a different value than the others since they're all ultiimately a similar topology and going through a similar power amp stage. If there was a difference in one of the modes that lead to increased volume on the real amp it might have been reflected in previous models too.
The MV taper on all the Mark IV models would have been adjusted the same way, so it doesn't make sense that changing them all from taper A to taper B makes just one of them louder.
That said, the increased volume might just be the way the model works now because of some of the algorithm changes vs what the mid-gain switch actually does.
But in a similar manner, if Friedman V1, 2, and 3 were all the same level, then in an update V2 was louder despite the voicings change being at a part of the circuit where later gain stages are capping the volume off anyway, then it would also seem worth pointing out in case one was accidentally louder.