That shouldn’t happen if you are using lightmass, but if you are using all movable dynamic lights especially the skylight then yes you will most likely run into this situation, The moving skylight does not have shadowing in real time it just uses the SSAO to fake it and naturally it wont make those dark areas for you especially with the AO since its just screen space and not accurate ambient shadow like offline renderers.
If you use the stationary skylight light with lightmass you should be fine. I would also check exposure settings, the default one should already work if you need to test it first. Finally make sure you don’t have any ambient reflective areas, and as suggested in the above comment make sure you place the reflective sphere inside the cave or adjust the position until you find that its working.
A good rule of thumb is to remove all textures and create a simple grey material first with specularity 0 and roughness 0.8, apply it on everything and check lighting, later increase specularity to 0.3 to 0.6 and check again.