I know this bug may be a duplicate, but since this is still in 1.14.3 pre-release 1. There is no way that this has been resolved yet.

I have a mob farm that has zero light below it, but during game play while unloading the chunks and then reloading them will create a light source on the mob farm with no real source is nearby... By exiting and clearing the cache of the world and then reloading it... it fixes it temporarily, but it will happen again after a while.

I attached what it looks like with the unknown light source and a screenshot of the same farm after I cleared the cache.