It is not displayed and remains invisible with adapta-maia or adapta-nokto desktop themes.
to show applet cornerbar:
select mint or cinnamon themes
in system configuration > themes > desktop > mint-Y-dark (for example)
PD:Checked: In a MV of linux mint 21.1 cinnamon the same thing happens, when applying the theme adapta-notko the corner bar disappears from the panel, perhaps an incompatibility of the theme so that the author of it is informed.
Moderator edit: It’s a Cinnamon applet, not an email address