OK, turns out that the UE4 automatic lightmap UV unwrapper indeed produced that error, although the unwrap seemed alright at first glance. What really threw me off was the fact that things looked fine in the “sunlight”. Changing the viewport to “Lightmap Density” was helpful in seeing that something is off - I should’ve thought of that earlier. I provided a second UV channel with a manual unwrap and all is now fine.
Thanks for the help!