I grabbed some stone from a furnace and place it in my inventory, but for some reason it wouldn't stack with the other stone I had in my inventory, as if they were different items, despite both being the same thing. I took some more stone from the furnace, and that stone wouldn't stack either, so now I had three separate stacks of stone in my inventory, even though none of them were full and I should be able to merge them. The same thing happened in a chest when I put it there. This happened to me again, with wood this time. (I'm using the new inventory system like on W10 edition, not the old Pocket Edition one)

In the screenshot, the 3 slots to the right with stone won't merge and stack into one.