Wow, amazing work!
Indeed because Stalin felt that because he wasn't notified of this uprising, those responsible could prove troublesome in the future conquest of Poland. So his forces were held back.
He wouldn't even allow Allied aircraft flying in supply drops to refuel in his occupied areas!
Once Hitler saw that Soviet forces had halted their advance he diverted troops back into Warsaw from the front for his goal of complete annihilation of the city.

Such a sad and horrible event.