Formatted view - hideColumnHeader not working in list webpart

I have a list webpart on a modern site which has a view formatted with some JSON

The list column header displays even though the property hideColumnHeader is set to true (this just started happening - was fine before)

It is OK on the list page -AllItems.aspx

but on webpart page list column header displays

I am using https://docs.microsoft.com/en-us/sharepoint/dev/declarative-customization/view-formatting#get-started-with-view-formatting to test

anybody have any ideas?