One reason a PRC border policeman might bring up for refusing to recognise a naturalised ROC citizen is the Chinese Nationality Law (1980) explicitly states China does not recognise dual nationality. So a naturalised ROC citizen who resumed his original nationality after becoming an ROC citizen would need to keep that quiet.
One way they can find out is your China ticket being in your ROC passport / TBZ name but your ticket in and out of the country from which you are entering China being in your other passport name. Or they might have a look at your ROC passport and wonder how you keep leaving and entering, but never arriving anywhere else. Anything could happen, I suppose. They might alternatively take the attitude that anyone the ROC defines as an ROC citizen automatically becomes a PRC citizen by virtue thereof and look no further than that.
My guess is that almost all of them are idealists who stayed after the revolution and their children. There were a good number of leftists in China who stayed. Most of them had a horrible time during the cultural revolution, but there are still probably a few hundred around. I met one such guy who gave up his British nationality to become a PRC citizen long ago. He was a pretentious twit who was at pains to show just how Chinese he was. His Chinese wasn’t even that good, or at least not as good as would be expected for someone who had lived in China for 50 years.
I can understand why some folks might be willing to renounce their born nationality to become an ROC citizen. I’ll never be in a situation where that would be an option, and if I did have the chance I doubt I would do it. The ROC is an alright place, though, and it seems that a non-Chinese can become a citizen and do their thing there without too much hassle. I can’t for the life of me fathom why any sane person would give up citizenship from any first world country to become a PRC citizen. Any time a read about non-Chinese who stayed after '49, and especially the ones who became PRC citizens, I wonder about their sanity. The one guy I met and the few others I’ve read about all seemed like people with something to prove.
It might also be woorth remembering that Taiwanese are rquired to complete the immigration forms in chinese characters, i fell foul of this requirement the last time we visited with my daughter.
So if any naturalised people can only speak mandarin and no ability to write then that could also be a problem.