I have the same issue with the same adapters. I took it a step further, and hooked up two PL2303 adapters to each other, and then I hooked up a PL2303 to a CP210X and ran a bit shifting test.
I posted the results in a stack overflow page: http://stackoverflow.com/questions/16832281/strange-bit-shifting-when-pl2303-connected-to-cp210x I hope you guys can shed some light on this, maybe even find a solution.
