Screenshot here: https://imgur.com/a/uM29KiN
It happened twice, and it allows mobs to spawn anywhere (a creeper spawned right in my well lit house). It happens when alwaysSetupTerrainOffThread (an option from forge-client.toml) is set to true. I couldn't upload debug.log to neither here nor pastebin.
Nevermind it's due to 1.14's new lighting engine.