The bug

Blocks that emit light do not receive smooth lighting, so blocks that emit low light within the radius of a higher light source are not smoothly lit.

Steps to Reproduce

1. Install the attached resource pack below
2. Navigate to a dark area
3. On the ceiling, place a platform of Respawn Anchors
4. Below one of the Respawn Anchors, place a lantern
5. Place one Glowstone in each of them

Here, the bottom face of the respawn anchors will not have smooth lighting.