It is a strange error. The default sbuild solver is unable to install the backported version of libtgowt-dev, because it sees the package in stable with higher priority.
I rebuilt the telegram-desktop package at Debusine runners this night. They pass --build-dep-resolver=aptitude flag to sbuild. The same does buildd, it uses Aptitude for solving dependencies in backports. Here is build log: https://debusine.debian.net/debian/developers/artifact/3123809/raw/telegram-desktop_5.7.2+ds-2~bpo13+1_amd64-2026-02-04T19:09:11Z.build
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