Hi there,
While I am working in the master layer, once I activate a mask layer by turning on the eye icon,
it does not show the light coloured glyph of the mask layer.
It was working before, but suddenly not anymore…
What do you mean. What “eye” icon?
Thank you for the reply!
I mean the small, blinking “eye” icon in the layers palette for turning on the layer preview.
For example, you have several copies/drawings of the “A” in Regular listed in the layers palette and when you turn on the preview of one copy, it is supposed to show the grey contour of the copy in the background.
For some reason, I can’t see it anymore. Do you have a clue what to do?
Hope, everything is explained clearly this time 
Can you send a screenshot of the whole window with the eyes enabled? (You can send if as Direct message).