I have no knowledge of any playfield manufacturer's processes, so this is not a knock on them... just some things I have learned from painting cars over the last 35 years.

There are many types of clear out there with different properties of curing times and final hardness, but they must be properly applied.
Improper application can cause any coating to fail.

The clear must be thinned to allow it to be properly atomized to be sprayed, and then to flow out and level on the surface.
You should use a thinner suited for the ambient temperature at application time.
Ideally, you want the clear to go on, level out, and then stop moving.
If you use a solvent that is too fast, it won't level out, and you will have orange peel.
If you use a solvent that is too slow, or you use too much solvent, you will have runs and sags.

One of the main causes for "soft" clear is not allowing enough time between coats for the solvent in the first coat(s) to evaporate out.
The top coat flashes off, trapping the solvents in the lower coats, resulting in poor physical properties of the finished product.
This can be fixed, but it requires removing (wet sanding) some of the soft clear, allowing a little time for any remaining trapped solvent to evaporate, and properly applying more clear.

Hope this helps...