Hi and welcome to Arturia forums.
IMHO then the default setting should be 0.500 for a A=440 Hz tuning. But if you take the single template preset, then it's actually set to 0.512 by default. If you it thru a tuner, then i actually think 0.517 or 0.518 is more correct.
Matrix 12 V is'nt the only Arturia synth, that in the center setting is a little flat in tuning. Things like this have been reported for years, and i think Arturia should make center tuning positions the default setting fro A=440 Hz.
Keep in mind, that you need to repeat playing notes, as the tuning change for each note. This i assume is because of Arturias TAE technology, that emulate analog behavior. You will never get all repeated notes to be excact, as they will and should allways vary a little as it's natural behavior.
Also keep in mind, that when you start to detune and modulate things that affect pitch, then a correction of the master fine tune can be needed. But it can also be wanted to have a sound to be a little out of tune.
Use your ears, your musicallity and a tuner to find the setting you find is best.
