It seems the provided pidgin icons override pidgin's own tray icon, and thus prevent it from showing the state in its tray icon (like whether there are unread messages or not). It will only show the static theme icon, with no obvious way of disabling this behavior.
The only workaround I found is to delete all pidgin.png files from /usr/share/icons/Mint-Y/apps/* and update the icon cache, which does not seem like the ideal solution.
I am using Mint 21.1 (though this was already the case in 21 and 20 IIRC, so it's not a new regression).
It seems the provided pidgin icons override pidgin's own tray icon, and thus prevent it from showing the state in its tray icon (like whether there are unread messages or not). It will only show the static theme icon, with no obvious way of disabling this behavior.
The only workaround I found is to delete all pidgin.png files from /usr/share/icons/Mint-Y/apps/* and update the icon cache, which does not seem like the ideal solution.
I am using Mint 21.1 (though this was already the case in 21 and 20 IIRC, so it's not a new regression).