Ya Zi had a hidden agenda!
Jiang Feng could not verify if it was really as Ya Zi had said, that it owed Donghuang Taiyi a favor and was willingly guarding the Shifter Emperor's Planet.
But he knew that Ya Zi's final quest was extremely suspicious
Perhaps, Ya Zi was not repaying Donghuang Taiyi's kindness. Rather, it was sealed here by Donghuang Taiyi and was forced to agree to do all of Donghuang Taiyi's request and to test the new Inheritor of the Shifters.
If the latter was true, him breaking the stone tablet would bring ruin to them all.
Ya Zi Will Reciprocate, repaying kindness with kindness, violence with violence. If Donghuang Taiyi sealed it here, then Ya Zi would be filled with hatred and vengeance.
Once it had been released, its power as a Primal Divine Beast could easily destroy the Ancient City of the Shifter Emperor that he had built.