The icon should be light in the system tray
A dark icon appears in the system tray
Operating system: Arch Linux (x86_64, recently updated) + KDE Plasma 5.18.3
Version of Telegram Desktop: 1.9.21 (Linux 64-bit binary from GitHub, installed in home directory)
Used theme: Day
Screenshots:
On Breeze Dark theme

On Breeze Light theme (everything is fine here)

Please, report icon pack bugs to icon pack, not to tdesktop
https://github.com/KDE/breeze-icons
@Inlifeuser I've reported it: https://bugs.kde.org/show_bug.cgi?id=418996
You can personally solve this by substituting the dark icons with the light ones.
sudo cp /usr/share/icons/breeze-dark/status/22/telegram* /usr/share/icons/breeze/status/22
sudo cp /usr/share/icons/breeze-dark/status/24/telegram* /usr/share/icons/breeze/status/24/
Remember to make a backup first.
the fix commit was remove the breeze-icons for telegram.
So if you dont want wait the update then remove/rename this files and the original telegram icon will back.

/usr/share/icons/breeze/status/22/telegram-panel.svg
/usr/share/icons/breeze/status/22/telegram-attention-panel.svg
/usr/share/icons/breeze/status/22/telegram-mute-panel.svg
/usr/share/icons/breeze/status/24/telegram-panel.svg
/usr/share/icons/breeze/status/24/telegram-attention-panel.svg
/usr/share/icons/breeze/status/24/telegram-mute-panel.svg
https://github.com/KDE/breeze-icons/commit/309ea93012f3fe990625419bcca9c11915ebc8ad
Most helpful comment
@Inlifeuser I've reported it: https://bugs.kde.org/show_bug.cgi?id=418996