OK so I had a look at this, @techAPJ is right, this feature has always been about [calendar][/calendar] never about events.
There has never been any support of showing events in categories. I’m sorry I know this is a little bit confusing as we kept everything in one plugin. But calendar and events are two different things.
I might add support for showing /upcoming-events in a category header at some point, but that’s not the priority ATM.