Doesn't make sense to me either.
All indications are that the basic Su-33 is docile - *remarkably* docile, in fact - at low speed. There are at least two videos showing pilots ham-fisting bolters on the Kuznetsov that are pretty instructive. In both cases they end up in rather wild flight attitudes that would likely qualify them for a ride in the bang seat in most other aircraft, but actually recover quite easily.
It is possible that SAC bungled the switch to an indigenous digital FBW system (if that was done), but in that case why not revert to the apparently robust and mature analogue original? If it comes to that, what makes the PLAN think the same outfit would then do any better with the J-31's (digital) FCS?
...
You seem to assume that the Su-33 uses the exact same fly-by-wire system as the Su-27s they sold to China. When that might simply not be the case. Even if they use the same hardware, the software could have different tweaks in it for sea conditions which would result in a rougher ride quality on a land based aircraft but are necessary on a carrier aircraft.
The Su-33 could also have had further modifications from the prototype the Chinese got from Ukraine that we are not aware about. Remember that the Russians said back then that simply copying the prototype would not be as nearly as good as the Su-33. I am sure the Chinese probably improved the prototype with several of their own modifications but it still needs more testing in actual flight conditions. I think it is a matter of time until the issues are sorted out.