If you use an anvil to combine an empty map with a compass to create an empty locator map, the resulting map will not stack with empty locator maps created by other means including:

• 8 paper + compass on a crafting table
• Empty map + compass on a crafting table
• Empty map + compass on a cartography table
• Paper + compass on a cartography table
• Obtained from Creative inventory

Maps created by any of these other means will stack together; only empty locator maps created on an anvil are affected.

Steps to reproduce:

1. Obtain one or more Empty Locator Map items from any of the means listed above and place it in your inventory..
2. Place an Empty Map item and a Compass item in the slots of an Anvil.
3. Shift-Click the Anvil's output slot.

Expected Results:
The new map is added to the slot in your inventory that already contained an empty locator map, creating a stack.

Observed Results:
The new map is placed in its own slot and cannot be added to the other map manually.

Cause:
Using an inventory editor, I determined that when created on an anvil, the resulting items are given a Tag: "Repair Cost: 0". Items created by other means do not have this tag, and when I used the editor to remove the tag I was able to stack the maps together in-game. Furthermore, I don't believe it was ever intended for maps to have a prior work penalty, and the related issue MCPE-18547 (which was fixed) seems to corroborate my belief, so I think the solution is to avoid adding the Repair Cost tag to map items.