Some applications cannot be backed up

For example, Telegram cannot back up data, but Telegram X can.

I vaguely remember that the app dev sets android.backup or something in the manifest file or something to false or not allowed or something similar.

Edit: It is the app. The OS can't do anything. I could be wrong, I am not sure.

It's on app devs to make their app compliant to Android's backup requirements. SeedVault has little to do with it.