There must be some faults with the design?

There are a couple of minor design issues. Whether you call them faults is a matter of definition.

Cut seat mesh
Unless the seat mesh is stretched tight and if the seat is tilted back, the seat mesh can come into contact with the rear wheel. This burns a cut in the mesh.

The fix is very simple. Get rid of the factory seat lacing and replace with cable (zip) ties. Using zip ties gives control over the mesh tension - you can have a very tight back mesh and a slightly softer seat mesh.

To completely eliminate any chance of mesh to seat contact, fit a plastic fender (mudguard) over the rear wheel. The clip on DF variety work well. Cut off the clip at the bottom that normally clips over the DF seat tube. Drill a couple of holes in the plastic and zip tie the fender to the frame cross member in front of the rear wheel. At the top, throw away the sliding metal clamp, drill another couple of holes and zip tie the fender under the crossbar between the seat tubes.

Front brake nipple
Depending on the model and year, the P-38 may be fitted with a caliper brake behind the front fork. If you allow the fork/front wheel to rotate too far to the left, the nipple adjuster on the brake arm will contact the lower frame tube. This may snap off the adjuster thread. Everything still works but it's annoying.

The wheel would never rotate far enough when riding the bike. I've had it happen when placing the bike on a rack. The steering has flopped over far enough to cause the brake to foul the frame.

I haven't had any other issues.