Bloom explodes when using realistic sunlight intensity values for directional lights in Lumen. In this example I used 75k, because that’s what the SunSky actor is set to in the plugin that is bundled with Unreal.
This appears to be specific to Lumen, as I tested the same scene with baked lighting via GPU/CPU Lightmass with normal results:
In the Lumen map, all lights are moveable. Lumen final gather set to 2.
In the Lightmass maps, lights are stationary except skylight which is static and Lumen is disabled via post process.
Everything else is the same.
HDR debug, not sure if this is just another symptom of the issue or the cause but the pre-exposure value is extremely high compared to Lightmass:
