Here are your parts
Replacement parts for MAH4000 AWW models | AppliancePartsPros.com

This should help it covers a number of Neptune models
16010061 Maytag Neptune Front Load Washer Service Manual - ApplianceDigest.com
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Could it be the water dispenser valve?
Yes it could.
Swap the wires for the hot and cold valves.
Now give it a try if you now get hot water into the unit your problem is else where.
OR
You could just check it with a meter to see if the coil has continuity, if not it is toast. I do this first as it is sefer than a live test.
If it has continuity then check if it is getting voltage (120 volts AC) when it should. If it does have voltage but will not open it is toast.

It tells you that the hot valve itself is OK.
Since you knew that the cold was working you knew that the machine is giving voltage to the cold. Now you know that the hot valve will open when voltage is applied so the valve itself must be OK.
Therefore the machine cannot be giving voltage to open the hot valve.

Put the wires back where they should be.
Then try the diagnostics (last manual in the download) to see if you can force the hot water valve to open.
If it opens then the problem is probably on the control board.
Check the board for any signs of overheating or damace.

If it does not open.
Unplug the unit and check that the wires to the hot valve have continuity.
Undo both wires and check the wire:
WH11 has continuity to WH11 on the cold valve.
OR7 has continuity to P7 pin 1 on the Control Board.

Check the thermistor by disconnecting P2 on the control board and then measuring between pins 5 and 6.
For the life of me I could not find any info in the manuals as to what it should be. I am guessing around 10,000 ohms at room temperature.
As temperature rises resistance drops on these devices.
My thinking is that perhaps the unit thinks the water is already very very hot and therefore is not opening the hot valve.
I will do more searching to see if I can find/confirm what the resistance should be.