A piece of the map is not merged correctly with the rest of the map

# Describe the bug

A piece of the map is place in the world that doesnt connect properly to the rest of the map (English is not my first language, cant explain myself better)

# Steps To Reproduce

1. Create a new character (i used Island pison layout B, got it in both attempts)
2. Reveal the map with debug
3. Find a square on the map with a few farm buildings that looks out of place
4. (Optional) download my save files. Im attaching 2 of them. One is much worse then the other - offending piece of the map cuts into the river

# Expected behavior

There should be no borders like this between forests and other things

# Screenshots

Screenshot


# Versions and configuration

- OS: Windows 10
- Game Version: 11495
- Graphics version: Tiles
- Ingame language: English
- Mods loaded: only core game

# Additional context

In both my examples part of the map that looks off is exactly the same, so im assuming its something like a Mall, but bigger and with forest. Still, it should at least not cut into the river/lake

Archive with 2 save files. Used Island prison layout B as a scenario
[save.zip](https://github.com/CleverRaven/Cataclysm-DDA/files/6049036/save.zip)

Load the attached save.zip files and reproduce the issue with Island prison layout B, using debug to reveal the map and inspect the farm-building area near the river or lake. Trace the map-generation path responsible for merging this map piece with surrounding terrain; done means the generated map no longer has the reported forest or terrain borders cutting into adjacent areas.

Written by the indexing model from the issue text.