You could go to the Microsoft 365 admin center->Groups, check if the group have a mailbox. Make sure the group has a mailbox.
You could try to create a new Group, then add a group calendar web part in the page. See if it works.
Use google chrome developer console tool and see what css attributes are being applied on any one row. I think of below possibilities.
Color of your front text is same as background(white or gray based on alternate row)
text are being hidden due to css attributes and displays only when on hover.
check for any css customization being done.
Definitely it ...