When returning to a world that was lit prior to 1.14 with various lighting forms (torches, sea glass, jack o' lanterns, etc.} rendered light sources will display a light effect for the block they are attached to, but will not produce light to the attached block or surrounding area.  Mobs will generate even though lighting sources are placed.  Bug is inconsistant, but will repeat even if all lighting sources are replaced.  Tends to happen to me while returning to the world during a night cycle.