I still have my 84S (collecting cobwebs in the basement). I never found it unreliable but it flung brass further than any other gun I've ever owned plus it put a nice half moon crease in each case making them iffy for reloading which was why I bought it. At that time (early 90's) boxer primed 7.62x39 was very expensive and virtually all of the imported stuff was corrosive (which I hate using) so an AK shooting cheaper, reloadable .223 seemed like a good idea at the time.
I can't prove this but I think the problem was the Chinese just swapped the barrel, bolt and mags from 7.62x39 to .223 but didn't bother to alter the gas system. Since the .223 has considerably higher chamber pressure than the 7.62x39 the result was faster bolt movement causing more violent ejection. I intended to experiment with a heavier recoil spring and try altering the gas system but then the gun went prohib under C-68 so it became a moot exercise.