What you are seeing here is expected.
When you are using a box, it is basically surrounded by light which passes only a small distance through the volume. That's why it's brighter overall, and in turn makes the fog effect easier to see when it is in front of other objects.
When you are using global override, the fog is taking much larger volume (it is like using a HUGE box) and the lighting is evaluated based on the environment distance ("enviro distance" in development/experimental stuff rollout -
https://support.chaos.com/hc/en-us/articles/4528469959953).
If you make the box object bigger OR if you lower the enviro distance value, you should get similar results.
Also there is a difference between rendering global volume between Corona 6 and 7 and the explanation is:
This is a result of a fix we did. Previously the global volume was around the whole scene geometry up to "enviro distance" from it. This means that removing/adding geometry changed the global volume. Now it is around camera up to "enviro distance" from it.So to me, everything appears to be working as expected, and if there are some differences compared to Corona 6 - that's a result of fixing something that was not working correctly.