Mail icon in Taskbar is blank

Winver 22H2 build 19045. 3086
How can an icon be blank?
If I hover on the taskbar in the middle and move to the left, just before I hit the first icon (calendar) a blank square highlights and the word MAIL appears. if I left click the mail prog opens.
There is a MAIL shortcut on the Desktop and I can pin/unpin to the task bar from there but the icon does not appear in the taskbar, just a blank square.
I have cleared and reinstated all icons in the taskbar.

yeah tried reboot - no change. also tried rebuilding icon cache - nope.
i have also changed the desktop mail icon to a loudspeaker but still didn't display.
edge, calendar and file expl pin & display ok - just seems to be mail prog affected.

Uninstalled Mail & Calendar, rebooted and re-installed. From the Start Menu, Calendar pins ok but trying to pin Mail has no effect ie does not pin to taskbar- weird!

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Thank you both for your input by the way. I have splinters from scratching my head.