Ok so I'm only a HVAC tech and not an EE but know a little but about air conditioners and the ways they are controlled so hopefully can add to the general knowledge level a bit.
Is that photo of the actual motor in your machine or just one that looks like it, as things are probably a bit different.
The Panasonic one in the picture is DC fan motor and normally works off a 300-350VDC supply. They are not a brushed DC motor as inside there's a tiny little 3 phase motor (permanent magnet rotor) with a little inverter drive board encapsulated into the casing so if you pry the end plate off you won't see anything. The giveaway is the motor leads especially the connector with the isolation between the Red and Black leads, however the pinout is slightly different to what I have seen on other Panasonic manufactured motors used in a few different brands of air conditioners.
For the motors I have seen the lead connector pinouts are as follows:
Red lead High Voltage DC Supply (this would normally be 300-350 VDC assuming 220-240VAC supply)
Space
Black Ground
White 15V Internal control circuit supply
Yellow Speed signal from PCB (~1-7 VDC)
Blue RPM Feedback signal from Motor to PCB
The one in the photo just has the Speed and Feedback leads reversed.
From techricky's explanation your motors sound similar but I have never seen DC motors running off only a 35VDC supply, are you able to post a picture of the actual motor in your units (especially the data label) and of the connector?
The reason the fan motor is slower to respond / ramp up to its set speed on heating mode is probably because the temperature of the Indoor heat exchanger coil is not high enough. Normally on heating mode most manufacturers start the Indoor fan at a very slow speed and then slowly ramp or step up the Indoor fan speed as the Indoor heat exchanger coil warms up. This way it stops people complaining about the air conditioner "blowing cold / lukewarm air" whilst its running or when the outdoor unit has cycled off on heating mode and the Indoor coil has cooled down.
Try this for an experiment on one of your dodgy PCB's. Take the temperature sensor off the Indoor heat exchanger coil and warm it up to about 40-45°C and then try starting the unit on heating mode, I think that you will find the fan will ramp up much faster. Actually for what its worth you should also check the resistance vs temperature calibration of that sensor but somehow I doubt this
I am pretty sure the factory has made a mistake with the programming of the micro controller especially if they have sent you new boards which solve the problem. The other way possibly around it is to try to offset the heat exchanger sensor temperature so it thinks its a bit warmer than it is but that would be problematic as there will be safety controls in the micro controller logic to prevent the Indoor heat exchanger coil overheating during heating and freezing up during cooling.
Of course you have confirmed that the refrigerant charge is correct and it does not have a leak, but that's not really something I should be going into on this forum
HTH