Buying a new vehicle and building it/tuning it seems to fix the problem. I can successfully save the tune and does not appear to corrupt itself after but this means I have to write down every part and tune by hand in order to then spend money on a new vehicle and parts (which I shouldn’t have to do) in order to recreate my cars.
Its not a big deal I have the money to do it but this is not what I’d call a minor issue specially if more tunes are affected…
On PC I was able to re-create the same problem so it affects both platforms but the fix is also the same. Something with the update damaged the saved tune files and corrupted entirely the vehicles attached to said tunes. Which is why you HAVE to buy a new version of the vehicle to bypass this annoyance.