How does sync slicers work in a custom visual?

[AdrianCox]

I'm trying to make sync slicers work in a custom visual.  My custom slicer supports date range selection by creating an advanced filter, and works well on a single page. According to the documentation this requires nothing but adding supportsSynchronizingFilterState to the capabilities.json file.  This seems far too simple: when I switch pages in my report, PowerBI calls the constructor and update functions of my custom slicer afresh on each page, and I cannot find any information on the filter state passed to my visual in the options. The sync slicer checkboxes look like this:

 

But the jsonFilters passed in are empty:

No jsonFilters

Where should I get the data from?  There are further documentation hints pointing at FilterManager, but this has been removed in recent versions.

[AdrianCox]

Further details: the example timeline slicer on Github does work. It's a good deal more complex than my slicer, and I can't currently spot what difference might be significant.  To make things even more bizarre, occasionally there is data in the jsonFilter field. Here's my full capabilities.json:

{
    "dataRoles": [
        {
            "displayName": "Date Range",
            "name": "category",
            "kind": "Grouping"
        }
    ],
    "objects": {
        "general": {
            "displayName": "General",
            "properties": {
                "selected": {
                    "type": {
                        "bool": true
                    }
                },
        "filter": {
            "type": {
                "filter": true
            }
        }
    }}
    },
    "dataViewMappings": [
        {
            "conditions": [
                {
                    "Category": {
                        "max": 1
                    }
                }
            ],
            "categorical": {
                "categories": {
                    "for": {
                        "in": "category"
                    },
                    "dataReductionAlgorithm": {
                        "top": {}
                    }
                }
            }
        }
    ],
    "suppressDefaultTitle": true,
    "supportsHighlight": true,
    "supportsSynchronizingFilterState": true
}