I never bothered to try the RTXGI because it was my understanding that this was just how the effect worked; It raytraces a sparse grid and so the lighting detail is inherently sparse. This is what allows it to be temporally stable and still run at high framerates. The demo that they ran during Epic’s livestream really highlighted this issue: The indirect lighting is extremely flat.
With volumetric lightmaps you can mitigate this with distance field indirect shadows, but I don’t know if that is supported by RTXGI. You might be able to enable SSGI as that has its own indirect shadow effect, but again I don’t know if that will have much impact as supposedly it was enabled in the Epic livestream and it comes with all the problems every other screenspace effect have.