I don't know why Telegram is trying to load those plugins at all. The system Qt loads them all by itself, and of course it knows where they are. It probably has to do with this file: https://github.com/telegramdesktop/tdesktop/blob/dev/Telegram/SourceFiles/qt_static_plugins.cpp that should be disabled entirely. I agree the Debian "fix" is a dirty hack and not a real fix.