That's one of the most characteristic and consistent bugs in this whole game, and the one (or nearly the one, I think I brought it up on page 2 or 3) that I started this very thread for, the so-called black walls of death. Yeah, it usually means complex brushwork broke the boundary between leafs, like because of a rounding error... Things like tiny slivers, either overlapping, or there's a sliver gap, or weird angles intersecting, or things going off grid. And the culprit brushwork is usually right where the artifact starts so you know where to look. And the easiest way to fix it is usually to just rebuild the architecture around it with simpler, flush, and on-grid brushwork. That's a more descriptive way to just say "yeah, what he said".
Generally speaking you want leaf-sealing BSP brushwork to be flush, on-grid, all 90-degree or anyway not-sliver-angled simple affairs (maybe not always, but when there's no better reason not to, and especially around leaf-border areas where the sealing matters) and leave the slivers and wonky angles and detail-work to non-sealing architecture you convert to func_stats that just overlay on top of the sealing brushwork. Edit: A leaf is the area sealed inside of visPortals if you didn't know, but I think that's an early thing everyone learns.