I don't think that would have happened. Given the cost of tooling for the plastic boiler, and the need to spend that again for a tool suitable for casting a metal boiler (I don't know if it would work for both) is probably the reason it came out with the plastic one. In contrast the upcoming K-class has a metal boiler with plastic parts for details.The retooling would have had to happen in the early days just as happened with the TOR 32cl, however my thought is pretty well that the engineer may well have said the plastic boiler would have, or should be adequate for the task, given that it is a reasonably large locomotive. In saying that I doubt that the Chinese engineers are all that knowledgeable or maybe fussed about model railways, to them its a job to be done, & they get paid for it.
I did mention the Kclass as well as the 50cl coming with metal boilers, also they are coming with all gear driven wheels, which I am now wondering about more as a result of what Hollywood has said, & backing up comments from Terry.