Hello. How to fix this?
Lightmap resolution: 512.
This is a known issue.
You can find more information on this post on the AnswerHub: https://answers.unrealengine.com/questions/34702/bug-lightmap-color-continuity-issue.html
You can also find some information about this on the Forums as well: https://forums.unrealengine.com/showthread.php?8491-Baked-Lighting-Variation-Between-Static-Meshes