I've no idea. I just guess it. Maybe some guys from garmin can confirm.
There may be different bluetooth modules between Fenix, FenixChina, FenixTaiwan and Quatix.
I meet the same problem on bluetooth also.
I don't have a Quatix since it is not released in China. I own a FenixChina version which has a hwid 1676. I managed flashing it into Fenix or Quatix, both modified by rgntool or remain original hwid. And in neither case can I connect with bluetooth again.
I even was going to use a Ultraedit to modify the GCD or RGN, but I didn't know which string to change, hence I gave up.
Some Fenix Taiwan owners told me that their bluetooth functioned normally under Fenix Firmware 3.70.
Spoiler: link
http://www.5i01.cn/topicdetail.php?f=228&t=3598310

So I think the bluetooth of Fenix is same with FenixTaiwan. But that of Fenix, FenixChina and Quatix are different pairwise.