What you are seeing here is the "black highlights" issue described here
https://support.chaos.com/hc/en-us/articles/4528311915025 , but in a reverse situation. What most of our users consider a bug you would like to use to actually see the shadow of the object.
The issue here is that in reality if an object does not reflect any light (visible to reflections disabled), then it cannot cast any shadows, because all of the light is passing through it, nothing can block it. Since Corona is modelled based on physical world, "fakes" such as invisibility but shadow casting can be a problem.
In Corona, whether you see the shadow/reflection as "dark" or "bright" actually depends on the ground material's roughness and camera position. You can check that by positioning your camera to see both the expected shadows and a strong reflection from the environment (such as HDRI sun) and then changing the ground material's roughness value in IR and observing how the shadow/reflection changes.
Unfortunately, there is no easy fix for this. As a workaround, I would suggest using full tree models visible directly and in reflections, or disabling trees' direct visibility only (while leaving visibility in reflections enabled).
it's been working fine until Corona 11!
This statement is wrong. It renders exactly the same in Corona 10 or older and Corona 11. There were no changes in Corona 11 which would affect this behavior.