When you launch Telegram now. Does your Telegram icon appear on your your panel?
And check your hot corner show all windows again with TG running.
Ctrl+Alt+Down is the default to show the window selection screen of all windows what are open. Which should include TG.
These keyboard short cuts can be configured in System Settings / Keyboard / Shortcut tab
TG does appear on the panel, just as it did before.
TG does not appear with “show all windows” hot corner.
TG does not appear with Alt+Tab (not “CTRL+Tab” like I mistakenly reported earlier!)
Net result: no change to previous behavior…
It must be something in your system setting. have a look in effects and check Desktop Effects and Windows Effects to see what status they are at.