Upon loading a world that was updated to 1.14.4 in Minecraft, one chunk near the player that was also previously updated loaded in 1.14.4 got replaced by newly generated terrain by the world seed. Changes where replaced, and the terrain does not match, likely because the world was not generated in 1.14. Reloading the game did not undo this.

Possible causes:

1. When loading Minecraft from the launcher the time before, I double-clicked the launch button, causing two game windows to appear instead of one. One window was closed before the loading bar on the Mojang screen was complete. This is likely not the issue as it was not loading the world yet.
2. Sometime before loading the world, my computer randomly lost its internet connection. This is a single-player world, so this is likely not the problem.
3. Upon loading the specific world the time before, I did not allow all of the terrain to load before hitting "Save and quit to title", and I exited the game almost immediately after the title screen appeared.
4. After exiting the game the time before, I preformed a hard restart (to try fixing the computer's internet problem). This is likely not the problem since the game was already closed and the rest of the world data was not corrupted or altered.