Incorrect sky light lower than 15 despite the sky not being obstructed

Created this issue as requested by this comment from a moderator.

After upgrading my singleplayer world from 1.14.2 Pre-Release 4 to 1.14.2 I noticed a few dark spots on the ground after going to trough a nether portal to the overworld. The sky light values are lower than they should be in these spots. Flying up in the air shows several subchunks higher up having a sky light value of 0, despite not being obstructed by any blocks above.

Here is a video showing this, as it's hard to accurately describe in words, and screenshots don't show everything there is to see.

I (currently) don't have any steps to reproduce the issue. (I currently don't have the time to spend.)

I've attached the region file where these lighting issues are present as well as the world's level.dat. If anything else needs to be uploaded please let me know.

Link to the world from before upgrading it to 1.14.2: https://www76.zippyshare.com/v/oe5niLUA/file.html
After upgrading this world to to 1.14.2 (again) and going through the nether portal at -46 55 -65 in the nether similar lighting issues show up. Not the exact same though.

Render distance 12.