All solid blocks can pass light when converting a world from 18w16a to 18w20 +

This issue affects 18w20c as well, and only in old chunks. Caves in newly generated chunks are fine. I added a spectator mode screenshot of chunks generated in 18w16a. What's really weird about this issue is that it's seemingly random. Some parts if the caves have 0 light (as they should) while with others pulling up the debug screen will show the light level as 15 sky, 0 block. If you go to one of the dark spots (0 sky, 0 block), they will light normally and update once light sources are destroyed. The light spots change with the time of day, and will always be 15 sky, 0 block, regardless of the distance from the surface. Interestingly, the light level of the light spots can be decreased slightly by placing new solid blocks overhead (the sky light level will update to between 11 & 13)- this only occurs for one layer of overhead blocks, the light level does not decrease again for subsequent layers. 

I believe this issue is also related to weird lighting on the sea floor: MC-128299. I've attached a screenshot of an unlit area of the ocean floor- as you can see, some areas appear to be lit while others do not. The "lit" areas show the sky light level to be 15, and the dark areas show the light level to be 0, just like in the caves. (Ignore the air pockets on the ocean floor, this is an unrelated issue: MC-129625) Like with the caves, newly generated ocean does not have this issue.

To add to Rat Dog's comment above- If you go and stand in one of those dark cave, log out, and then log back in- the cave within chunk you are in will now be lit up.

Affects 1.13-Pre1 too. Really hope this is fixed for 1.13's main release!

Please fix. My game is messed up because of this. :/

If this works from 1.12.2 -> 1.13-pre3 we are going to have to consider this bug fixed.

It is so strange that some people are still experiencing this issue, Hmm..  My 18w16a world works perfectly after being updated to 1.13.  Don't know what would cause a difference with some worlds vs others, perhaps world type or generator options maybe?

Can anyone confirm for 1.13.1-pre1?

Still happening on ps4

Java Minecraft isn't available on Playstation 4. Please make another report, if you're experiencing issues for Legacy Console Edition.

I was still experiencing this issue in 1.13.1, but it is definitely fixed in 18w43a.