Integrated GWC is unable to find a layer belonging to default workspace, when the OWS request has no mention of workspace as part of layer name or inside the request URL.
Steps to reproduce:
Enable GWC WMS Integration in Geoserver
Enable Tile Caching on a random layer
Make a WMS request with Tiled=true param set and BBOX matching GWC gridset. (Can be obtained by previewing the GWC layer)
The HTTP response headers will show
geowebcache-miss-reason:* not a tile layer*