So I have a very big issue here and it has been bugging me out all the time
Every single time I convert my 1.13.2 world into a snapshot update, any blocks or torches that emit light would not produce light and now mobs are spawning all over the area. The first picture is from 1.13.2 and the second picture is from snapshot 19w14b. Please help. Thanks
I believe the lighting engine changed in 1.14 at some point, so you will need to manually go around updating each light source. I don't believe Minecraft has any debug command to force lighting updates and F3+A (which force-reloads the world) or relogging doesn't update lighting so those options won't fix this.
I went through this exact issue in 1.12 with the Phosphor mod, which specifically rewrites Minecraft's lighting engine to be more efficient. It claims to be highly compatible with most every other mod, but the net result of using it for me was that light stopped propagating downwards...any block that was between hills or slopes or elevation change of any kind went completely dark even if there was a torch there already.
I wish lighting would self-update like water does, it was extremely annoying to have to travel through my entire world to reset the dark spots after I removed the mod.