This issue occurs with all light sources, torches, redstone lamps, glowstone, ect. Problem seems to be limited with individual chunks, but chunks with this problem may be adjacent to one another. I'll leave the chunk by any means (elytra, logging out, nether portal, walking away) and upon coming back certain areas will be unlit and the light source will only be emitting a glow limited to the block it is on. Unsure if it's only affecting certain, specific chunks or every I come back it's randomized, From further testing I did notice that some chunks in particular tended to do it more often. I haven't taken note of that. Issue seems to effect ALL light sources within the chunk as well, regardless of Y level (all caves within the chunk that had been previously lit become unlit). Happens for all players on the server, seemingly random with the only consistent factor being that the chunks unload and then upon reload they are like this. The yellow X's in the screenshots show which blocks are at a light level in which mobs can spawn on. (Happens on both vanilla and modded clients, I just used this mod to put emphasis on the problem). Restarting the server with OR without "--eraseCache" did not help. World file can be provided if you'd like but it's 1.6 gigs.

edit: placing a transparent block (glass) next to a torch does not make the light source function properly again, where as placing a solid, like stone or dirt, does.