I first found my maps on a wall i have were rendering darker than normal as i walked up to them. I can not reproduce this to give you pictures etc but;
When i rotated a map in a item frame i seen the player or map marker change colour.
I rotated the map again and the whole map became darker.
Rotating again makes it back to how it should be.

Also I found when looking inside players inventory that the rotated maps changed colour when the mouse moves over items in the inventory.

Pictures added are 3 from inside players inventory. You can see the maps in the background have changed colour(become darker).
Another picture shows the same map in each rotation. Player/map marker is dark in one position, lighter in others. Map is dark in one position and normal in others.

What I expected to happen was...:
I thought the maps should rotate with no colour changing happening at all.

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...Open any map in 1.13 pre-3 
3. ...Place a opened map in a item frame and rotate
4. ...Open inventory with maps visible in background and move mouse over items.