Reproduction steps:
1. Open the attached project
2. Open the scene 'PreComputedGI', build GI in the Lighting tab and save (image 2)
3. Open the scene 'BakeGIScene', build GI in the Lighting tab and save (image 1)
4. Open Scene Additive 'PreComputedGI'
Note: Objects' in 'PreComputedGI' are not lit (image 3)
Note: Disabling 'Baked GI' in the Lighting tab causes 'BakeGIScene' to be lit, until lights are built (image 4)

Result: Loading a scene with Precomputed Realtime GI additively onto a scene with Baked GI does not display Precomputed Realtime GI