Additively loading Light Probes when using Realtime GI causes probe values to rest to ambient

Steps to reproduce:

1. Clone and update the following repo: [Internal link] 
2. Open the Scene TESTSCENES/MASTER.unity
3.  Generate lighting for the current Scene constellation.
4.  Save all Scenes to write references to Scene files
5. Look at the lovely red lighting in the red plane's light probes. So hot. So red. Mmmmm. [sic]
6. Unload the Scene Red_PlaneWithProbes from the Unity Hierarchy.
7. Load Red_PlaneWithProbes again.

Observe:
Light Probes that were unloaded and reloaded now fall back to the ambient colour coming from the Skybox. 

Expected:
As the Lighting Data asset has not been unloaded or replaced, these Light Probes should recover the red indirect they had before. Infact, if you rebake the Scene using BakedGI, this is the behaviour you get.

What's even more unexpecteder is that if you Save All after the lighting generation (Realtime GI) and then close the Scene. Repoen and then do the unload/reload from the repro steps above, you do get the expected behavior.

I can't even...