WMS on ArcGIS Server 10.6 does not show image tiles but tile footprints

I have set up a Map Service on ArcGIS Server 10.6 and turned on WMS capability for it.

When I tried to add this WMS to ArcMap 10.6 running on the same machine, it didn’t return the actual images but tile footprints.

I also tried add a WMTS service from ArcMap and this worked and showed the image files successfully.

Is it a bug or something I didn’t do correctly in setting up the WMS service?