I just bought an 11-year-old house with two widely separated bathrooms. Both shower stalls have the exact same Moen valves and symptoms. Turning on the shower produces really hot water in the center position, but only warm water in either the extreme right or left positions. Cold water is sometimes available for a few seconds at start-up, but that is obviously attributable to standing water in the pipe overnight.
I have replaced many Moen cartridges in the past, have never seen one do this. I checked under the house for a tempering valve, but there are only supply lines. If there is a tempering valve, it would have to be hidden behind the shower wall and I can't imagine anyone burying one there.
Well, duh. Maybe I should just install new cartridges and be done with it. It seems odd that both cartridges would fail at the same time.