Minimum IFR speed is going to be derived from a whole host of factors. I suspect for the 412, it has something to do with any of the following-
a) heading hold reverts to the yaw channel somewhere below this speeed
b) starting to go to the back side of the power curve, and very difficult to fly accurately at this speed repeatedly (I know it can be done, but probably not well or for long)
c) trying to maintain ball centered flight is difficult below this speed (takes a lot of sideslip to generate enough sideforce to move the ball, or put another way, you can be in ball centered flight and have any one of large variety of sideslips)
d) gains for the autopilot may not be optimized below this speed
e) a go-around using the autopilot go-around feature, which uses (if memory serves me right) 70 or 80 knots, would be a complicated affair from a speed lower than 60 knots, particularly if single engine
f) some other reason I can't recall as the caffeine hasn't kicked in yet....
Personally, I'm suspicious of helicopters with a minimum IFR airspeed lower than 60 knots unless they have some sort of super-airspeed system. If you're at 40 KIAS for example, there isn't enough dynamic pressure below that airspeed to tell you anything useful. So if a gust knocks you back below 40 KIAS, what have you got?
If we use a parallel to FW, their approach speed is 1.3 times stall speed. We don't stall, but if we use the airspeed indicator as a loose parallel then we should have 1.25 times the minimum airspeed that indicates reliably. The 'reliably' part would include at maximum lateral CG, and with the slip ball not necessarily centered - high sideslip angles will really screw up most airspeed systems. Just a thought.