Yes but in those countries it happened gradually, I cannot find any case of a 50% drop in just two years.
Yes the two-child policy being introduced I can see it dropping the birth rate to China's national average, but not well below China's national average. Since Xinjiang people naturally have higher fertility it should still have been higher than average even after the 2-child law was enforced equally.
Uyghurs marry and have children much earlier than Han. I won’t be surprised if many had already used up their quota when family planning implementation begun.
Also Xinjiang has practically been in a state of mass mobilisation for the past few years. Han and Uyghurs from North Xinjiang moved to South Xinjiang to implement new policies. Cadres were making weekend visits to local Uyghur families, which upended everyone’s lives. And also the uncertainties of the situation. Who wants to have children with all these going on?