Sharp lines across mesh, ruining normal map

I’ve been trying to wrap my head around exactly why some of my ceiling tile meshes in my level have sharp lines across them. The normal map applied to the material is just a basic noise texture, intended to give the impression of popcorning.

I’m certain it must be a light nearby causing this, but I don’t know if it is being clipped by a groove on one of my tiles, or perhaps it’s some sort of bounced light from somewhere else? My question: Is there any way to use the show flags in the editor to trace the source of this kind problem? Do you know of any other way to solve it? I’d rather not have to fiddle around with a bunch of things while having to rebuild each time (takes me ~2 hours to rebuild).


Also, I’ve already tried fiddling with most of the different lightmass settings, including uncompressing lightmaps.