I have discovered the root cause, and opened a separate report for it (because the bug is still present in the apt version used in trixie).
Filed under #1126140 ( https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1126140). Closed.
- Bug#1126054: /usr/bin/apt-get: Segfault when installing .d... Arvid Gerstmann
- Bug#1126054: Further information, with possible fix Arvid Gerstmann
- Bug#1126054: Root cause discovered. Please close. Arvid Gerstmann
