I have bumped into that issue a couple of times, most often changing the lightmap resolution to a really high value deals with it, like 1024…
But, if that’s not a good solution, try to break down your model.I know this seems like a terrible idea, but it might just work, as it did for me. When you have objects with many small faces, pieces, in the UV channel, like yours, you can break it down so you have more space for the UV’s.
I would like to add that i have done a small test to completely eliminate the cause to be my unwraps as everyone keeps on repeating.
I have baked the lighting using my own uvw and one time with the autogenerated uv’s from Unreal. Both uv are extremely different, meaning that if there should be artifacts, they should be very different.
Now here is the interesting find of this test, using either one of these different uv’s generates EXACTLY the same artifacts.
Thus, it is NOT my unwraps. My uv’s are NOT causing this!!!
But now the question is of course, then what the hell is causing this?