Hi, I am not good at English. I would appreciate your understanding. 
I am experiencing Instanced Static Mesh (ISM) / Hierarchical Instanced Static Mesh (HISM) lightmap errors in level streaming and lighting scenarios.
Please see the screenshot below.
In order to reproduce this:
- Set “Project Settings - Streaming - UseBackgroundLevelStreaming = false”
- Prepare BP with ISM or HISM.
- After you create a sublevel, you configure the sublevels to stream through the streaming volume.
- Place ISM / HISM BPs at each sublevel.
- Create a lighting scenario level.
- This lighting scenario level is configured to be loaded during playtime using a trigger or the like.
- After execution, load the lighting scenario level through the trigger and load / unload the sublevel.
- You can confirm that the ISM / HISM light map is faulty.
Note:
- It can be prominently displayed in dark environments where Directional Light does not reach. (Environment with a roof)
- The more objects placed in the level (large texture size), the more lag happens during streaming load / unload, and I had to set UseBackgroundLevelStreaming = false in my project.
- Only ISM / HISM has been confirmed to be a problem, and there is no problem with BP made from static mash or static mash components.
- Simple sublevel streaming without a lighting scenario is not reproducible.
- This is also reproducible in 4.19 Preview 4.
The test project download link is:
https://drive.google.com/file/d/1vZdSeXSST00eZL9FHmjgeRO-J5D5avEu/view?usp=sharing
Thank you for reading my difficult English.