(First bug report attempt, please let me know if there's anything I can do to provide better information.)

Most chunks, but specifically ones with villages, lose all block light data when unloaded. This means that after a player logs off, or returns from the Nether, all light blocks, such as torches, lamps, and jack-o-lanterns have a block light value of 0 (as seen from the F3 data screen under "Block Light: 0")

Reproduction:

1) Place light sources, such as torches, in a general area, spanning multiple chunks.
2) Summon entities, specifically villagers, into this area.  Anything more than 25 seems to begin to cause the issue.
3) Build and enter a Nether Portal, while no other players are in the area (forcing the chunks to quickly unload).
4) Upon return to those chunks, especially be returning through the portal, the Block Light value in many chunks will now be 0.  (Set time to night to easily locate dark areas) 

Notes:  I have spent nearly a week investigating this.  I cannot find a mention of this online, and cannot find a way to prevent this from occurring.  I have updated the Spigot server via BuildTools whenever a new version is available (checked through /about in-game) hoping for it to be fixed.

We can reproduce this quite consistently on a public server, should someone who thinks they can help need to see this.  Been using Spigot for years, and I'm at a complete loss.

Random-light-updates also does not seem to activate from any configs, and has yet to have any effect on this error.

Does not affect vanilla 1.14 server nor client, not reported to Mojang