Sorry for that I did not consider the situation of event list and introduced confusion.
I created a classic calendar list, then I converted it into a modern list. After that, I created a modern calendar view and add JSON codes for the modern calendar view. The same issue as yours.
I did lots of research, there is no official documentation to mention this issue.
And in my opinion, due to it was converted by classic calendar list, so JSON codes do not work on it.
I suggest you open a service request with Microsoft to get more detailed background information.
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