Observation:
In GWC Demos page reached through the GeoServer admin UI, I am attempting to view a layer group.
Some requests are working: https://serverUrl:8443/geoserver/gwc/service/wmts?layer=lyrGrp%3AOSGB&style=&tilematrixset=OSGBGridSet&Service=WMTS&Request=GetTile&Version=1.0.0&Format=image%2Fpng8&TileMatrix=OSGBGridSet%3A3&TileCol=0&TileRow=19
Others (most) give a 400 response: https://serverUrl:8443/geoserver/gwc/service/wmts?layer=lyrGrp%3AOSGB&style=&tilematrixset=OSGBGrisSet&Service=WMTS&Request=GetTile&Version=1.0.0&Format=image%2Fpng8&TileMatrix=OSGBGridSet%3A3&TileCol=3&TileRow=17
In fact for the whole of the UK, I am seeing only a few tiles in the top left of the extent.
Detail. OS: Ubuntu 16.04 Platform: Tomcat 8 / Geoserver 2.16
Using OS GB tiles EPSG:27700 Layer: Layer group containing 3 layers.
The issue can be seen in the Geoserver Demos page as well as through our application.