This could be a conflict with another mod or a custom livery. So try to remove all other mods or custom liveries from your community folder (temporarily). And try again. Also there could be a problem with your keybindings interfering with something,
I’ve been using the keypad. Every other field on the MCDU is fine. I can enter in other details however the CRZ ALT won’t take in anything apart from the number “2” as if I am cruising at 2ft"
Where it says CRZ ALT/TEMP, I can only enter in the Temperature portion in the same field starting with “/” on the keypad.
I have never encounter this weird problem with any of the developer version. Perhaps its conflict with other mod (e.g. liveries)? If you have removed all other mods/add-ons except A320NX mod and this problem persist, you might want to talk to the mod guys on their discord.