What I expected to happen was...:
After opening the map I expected it to fill like it always has: while discovering new terrain.
What actually happened was...:
After opening the map I've noticed it's completely blank and nothing can be done to fill it. It also appears that this bug occurs only when the map id is 0.
Steps to Reproduce:
1. Open any 1.13 world without maps created (eventually use a command to give yourself this item: minecraft:filled_map{id:0})
2. Take a black map and open it (If the filled map has id 0, it should be empty/blank).
Can't reproduce