Lightmap issue between 2 modular wall asset

Hello guys,
I have a question regarding light map. I have created a modular wall asset. I took the same wall asset and cut a hole in the top middle area.
But when I put the wall assets together, it create this very distinct shadow seam ( marked in red ).

Before I had the same light map uv as before and it has the seam, I then tweak the uvs and have snapped my light map uv to the grid but it doesn’t seem to help.
I’m wondering why with the same asset, but modifying the mesh cause such lightmap seam in lighting ?


Can’t see the attachment yet but it’s probably this issue:

Best fix is to combine simple meshes since it’ll fix that issue and it will improve performance by reducing draw calls.

Thanks for the reply,

I have updated the url, hope you can see the attatchment. I had this same lightmap issue when I didn’t know about the lightmap uv, and the lightmap seam is mostly fixed when I created the lightmap uv.

I didn’t know why putting 2 version types of wall ( 1 with the hole ) side by side will cause the seam again. Increase the lightmass quality doesn’t seem to fix it, so I think there is something I did wrong