For anyone that runs into this issue -
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Run a map check to see if you have any static mesh actors with null references.
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Look through static meshes that might have broken material IDs - you will know it’s broken by opening the static mesh and looking at the material slots. If there is a white ‘X’ to the right of the material, it means there was, at one time, an element that referenced that material that no longer exists. Click the X to remove it.
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Look for static meshes that have more overridden materials in the sublevel(s). This will show up during map check. Select the actor, ctrl+b to find it in the content browser, select it in the content browser. Inside the sublevel, right click the actor and choose to replace the actor with itself. This will reset the overridden materials. Then re-apply the materials.
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Check for any static meshes that have strange behavior inside the sublevel. Delete it or replace it.
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Re-bake lighting to see if photons are gathering.