This tend to happen with containerized apps (Flatpaks, Snaps, etc…). You could try to download Flatseal. It’s a flatpak app that lets you manage Flatpak permissions. There might be some settings there that could allow the you to set the Telegram app to access your DE light/dark settings.
In flatpak, if you go to global options
then go down to Environment you’ll see variables to apply to all flatpaks:
if you set all the variables as I have it above, you’ll never see a bright flatpak application again! Well, I haven’t, anyway!
I also noticed that this issue also prevents any theming of the packaged version of the app from vanilla, so it is more than simply dark mode not working.
If you need a Telegram workaround until the dark mode issue is sorted out in the packaged version then you can download the Linux tar.gz version of Telegram Desktop from the Telegram website using the Get Telegram for Linux x64 button on this page - https://desktop.telegram.org/
After you have downloaded the archive extract the archive contents [Telegram & Updater executables] into /home/<username>/.local/bin/ and run it from there, it sets up its running files into ~/.local/share/TelegramDesktop/
I have been doing this for a week or so without any problem, I also uninstalled the packaged version meanwhile to clean up the application menu as when having both installed the menu has duplicate entries.