Layer Mask issue

Am I doing something stupid here ?

My layer masks seem to have changed from the normal white/black rectangle showing the masked areas to an outline of the mask symbol. It seems to perform in the correct manner but I can only see a visual representation of the mask by selecting the properties which is highly inconvenient. This is the same whether I am editing a new image or re-editing an old one which previously displayed the masks correctly.

This is on my I-Mac running PS 21.0.2 on Catalina. Incidently the latest PS version on my Windows 10 laptop doesn't exhibit the same thing and works in the same way it always has and displays the usual symbol in the layer.

Any advice would be most appreciated.