This is just a cosmetic issue.
I have 10,000 Mistscale, this understandably takes up a lot of space in my UI if i don't have it set to merge stackable items. However, when I do the number is too large to be stored inside the tile (see image).

I'm not sure what happens when the number gets super long. Perhaps a solution would be as the number gets to certain lengths it could be formatted i.e. 10k instead of 10,000