That gear shouldn't be sticking out, when you put Greece in your driveshaft it will expand, but I got a hisun sector 550 that was put together wrong from factory. Mine had a spring behind the nut in the differential, only way to know is remove driveshaft and look at that gear, and make sure they didn't add a spring behind your nut, it will keep that gear from going all the way in. That's probably about the same amount of miles on mine before it happened to me, here's picture of bad parts, don't put the spring behind the nut if yours has one behind it.
kgs's post in 2019 Hisun Sector 550 Hard Starting was marked as the answer