In-game (note, it sometimes looks like this in the editor too, but the shadow artifacts go away after you fly closer to the static mesh affected, then doesn’t return)
If you turn on wireframe view, do the artifacts line up with the edges of the mesh triangles?
If the answer is yes then the second question is: Do the artifacts line up with the edges of the meshes UV islands?
If so, most likely this is texel leak from the higher mip levels due to insufficient padding, which is why it is only visible at a distance. Because the higher mip levels require more padding.
You can either regenerate the lightmap UVs with more padding, or what I would do is just make a better lightmap UV where the seams are better hidden.
Are you vertices between triangles welded/merged? Looks like they may not be, based on that UV image
If the geometry is fine then I think the next step would be to use the visualizers to view the buffer outputs individually so you can isolate where the issue is. Probably start with the Lighting only view:
If the lighting is the issue, you can use the show->lighting components menu to toggle each of the lighting elements off individually to try to further narrow down where the problem is.
I imported the exported version of my mesh into Blender and looked at its UVs. You are right, they are not connected, as I can freely move both triangles.
I do see a big difference. For these distant meshes why is increased padding needed, is it because of mips, if so then how would I go about changing it, via LOD settings in the mesh?
UPDATE: User LordFluffeh suggested I try increasing the bounding box of my meshes, which fixes the issue perfectly while making better use of lightmap UV space, but I do worry about the potential performance costs of this (no occlusion culling I presume, since the player is always inside of the bounding box and bounding sphere):
