The eye icon for each layer mask has disappeared.

In my Lightroom Classic (15.4.1), the icon for each layer mask located next to the mask name has disappeared. Previously, each mask had an eye icon and I could click on the eye which would turn off the mask visibility and change the eye icon to an eye with a slash through it.  Now there is just a dark gray area with no eye. 

The only way to turn a mask on or off is to control click on the mask and select hide which is clunky. Is there a hidden setting that will restore the eye icon? Thanks