Hiya. I don't often use stone (i.e., smelted cobblestone or un-mined cobblestone) but I just tried to stack it in my inventory. Here's the situation:

• I smelted nine cobblestone in a furnace and collected nine stone.
• I wanted a multiple of four so I added three more cobblestone to the furnace and collected three more stone.
• The three stone were transferred to a separate space in my inventory from the nine original stone.
• When I selected the three and tried to combine them with the nine, the nine and three simply switched spaces but did not stack.

I asked my husband (who uses Windows 10 PC, running 1.14) and he said his stone stacks now but did not in the previous update. 

Thank you!