!!! I have literally went into rage mode and I've started to change all the parameters from the developer rollout in Corona. Embree on/off, Pathtracer, VCM, Light Tracer, PPM, VPL, terminat shandling, adaptivity on off literally everything.
AND THEN... (David Gilmour song reference)
I noticed the setting "ENVIRO DISTANCE" !!!! I have lowered that to 500m AND I get the identical results regardless if there's a smaller plane or a big plane. Still the values seem messed up to me and this only works with the Distance parameter in volumetric scattering is higher than the enviro distance. I had to lower my ground plane at -300 meters otherwise my water would've been too transparent, and the planes would have very different refraction results (So the problem I had before but more accentuated). I have just discovered this so I am still looking into the numbers.
EDIT: overjoy too soon, at different angles there are differences again but smaller...I hope I can adjust them in post though.