When a phantom spawns through spawners or naturally (via custom biomes through datapacks), they ignore light level completely, while insomnia-induced phantoms use a light level requirement of seven or less. This often results in them spawning and immediately burning up.

They'll also spawn on the ground rather than in the air, but I'm not sure that actually counts as an issue.

To reproduce:

1. Get a spawner and place it in broad daylight
2. Use a phantom spawn egg on the spawner

The phantoms will spawn in daylight and immediately catch on fire.