The bug

A painting shown inside a spawner is dark.

Note: It appears it is world generation or position dependent whether the painting will appear dark. However, in superflat worlds at spawn this issue can be reproduced consistently.

Reproduction steps

1. Create a new "The Void" superflat world
2. At spawn, use the following command

   /setblock ~ ~ ~ spawner{SpawnData:{id:"minecraft:painting"},Delay:32767s}
   

   The painting appears dark

Side note: When teleporting multiple thousand blocks away from spawn (e.g. using /teleport ~ ~ ~10000) and using the /setblock command from step 2 above again, the painting is for whatever reason not dark.