It's possible that it's being caused by your lightmap's UVs not falling in a pixel. When your UVs aren't snapped to the pixels and they cut through a pixel, the lightmap sometimes bleeds like this because the lightmap is telling Unreal to cut the pixel in half, which it can't do. If this is the reason you just need to snap…
I think it's a simple case of making sure you've turned on pixel snap. could be wrong. Don't worry about a really tiny bit of distortion though. @atomander Glad you got it sorted, I always forget that lightmap resolution is usually first port of call.
Sorry to jump in. How does snapping LIght map UVS to pixels help with Organics I wonder? Like unwrapping an un even edged UV island. Should I align all the verts together in the UV editor even though this is incorrect workflow for Texture UVS, as doing this would distort and stretch the texture?