Nitro doesn't chemically bond to the surface. Notice the chips on any old guitar. It tends, in fact, to shrink and pull away from the surface, cracking as it goes

Second, this is a decades long process. Anything that happens over the course of a few years is something else.

What happens to some acoustics is that they tend to equilibrate with their surroundings. A guitar built in Bozeman will sound different after sitting a couple years in Miami. Humidity is king with an acoustic. For years and years we put humidifier tubes in them when not in use.

There is a world of difference between a "nitro" finish and a "nitrocellulose lacquer" finish, the later being the vintage method and the former being the modern. Many, if not most modern producers (Fender for example) use a water-based "nitro" finish (made by Sherwin Williams) for their guitars.

Chuck