Although previewed as being completely uncapped outside of programming limitations, it seems that the Cloak of Infinite Potential is rather finite, with a maximum cap of +200k stamina from threads.
As showcased by Redditor u/Maleficent-Till6391, the cloak appears to stop gaining stamina, even when using a specific thread to increase it. This limitation isn't exactly holding them back though, as their 685k total stamina is equal to about 13.7 million health!
Of course, everything needs a defined limit, and it's likely that Blizzard never anticipated the various farms allowing players to reach it this quickly or there could even be a technical limitation keeping stats on items from growing beyond a predetermined number.