I think everything was already explained in this thread. Corona doesn't "like" fake effects such as objects not casting shadows and they can lead to shading issues. There is at least one known workaround to this, but it is not perfect. Can you show your results? Maybe then it will be possible to come up with some additional ideas.
I've setup a simple scene trying to replicate the workaround but it doesn't work 100%: it does get rid of the black sun highlight, but then the backplate is not visible in the reflection anymore, as said @leo_surrealismo.
Is there a way to have both a normal highlight and the backplate being reflected?
See attached screenshots and scene (3ds Max 2023 + Corona 10HF2)