I'm seeing this too. I believe my version was created in 18w16a (maybe earlier) but was run under 18w16a for a while and then this 18w20a/b/c cycle.
Attached a snapshot showing in an entirely enclosed area with no sky access the light is set to 14 (from 14 sky access) even though the only lighting should be the 9 block light.
This is killing spawning except for underground slimes. I AFKd over night at a mob farm and got zero drops. Thing worked just fine until 18w20a (though didn't try the 18w19 cycle because of the loading issues from 18w16a)
Sorry about that too many windows... This relates to: MC-125900 and MC-129682