Hi! I have been working in Unreal for many years, but this is the first time I have encountered such a problem. When I bake the light, there are no bounces at all.
Everything you need is marked with static, everything is fine with UV, you can not offer these solutions, they have been tested for a long time.
The screenshot shows that there are no bounces. At the same time, the force of the bounces is multiplied by 100, and the brightness of the sun by 1000, but there is no effect.
CPU Lightmass or GPU Lightmass? What are your Lightmass settings? Do you have a Lightmass Importance Volume wrapped around?
CPU Lightmass. Settings is all default. Volume is placed.
Earlier it was fine, but at some point, i rebaked light, and found out that bounces is gone.
Strange. You could place a Point Light (Static) inside your walls and try to bake again? Just to be sure that it is bake’able on the inside. (could be a problem with normals, and so on …)
Tested with point light. Normals is ok. Anyway this is my model, and i am sure, its ok. I have done test earlier, if i migrate project to empty, it will start to work normally, but after some time bounces will be dead again.
Ok! Did you try to Clean + Validate cache on your Swarm Agent?
Yes. No changes.
Could it be the materials you’ve applied? Or are these still some basic, un-fancy UE mats?
If the project isn’t that big you could try to create a new project from scratch instead of migrating the whole stuff into another one. Just to check if it behaves differently to only migrating the whole bunch. But the fact that migrating solves the problem for some time really sounds buggy. Could be worth a bug report. But I’ve no further ideas anymore. 
Are you using a Postprocess volume? If so, try cranking the “indirect lighting intensity”.
I can’t move project easily, it is huge. No postprocess for now. Deleted every light from scene, except sun, so i can see clearly what happening, but no results…
Delete the sun too? Maybe some settings prevent the light from bouncing. Because, if the static point light you placed for a test is bouncing, it may be a good idea to re-create the whole lighting setup.
No, sun is only light source left. I think this is some settings, or maybe some asset breaking things… but how to find it?
Try to delete every light source. Then again, a new static point light inside would be good. Just to be sure about the behavior?
To check if some asset is intefering, you just start deleting a bunch … and then bake 
Try to start with bigger stuff like EnvironmentSpheres, Horizon objects and so on …
have you marked your light as stationary or static?
on the world settings or in project settings have you allowed precomputed lights?
is your geometry static, or at least in the shades options have surface shadows?
all this may affect your project, but I’m sure at this moment you already found the solution, just in case.
I had the exact same problem. Solved it by setting “generate lightmap UV” on my meshes and removing “UE4 compatible lightmaps”. I was trying to use default cubes from Unreal, but they don’t want to work properly with GI for some reason, so I copied them and did these modifications.
