It appears that postgresql-15 packages pulled for minor releases before 15.4 in bullseye, only. That's very specific. Was it a mistake or a platform-specific (bullseye) issue?
That's normal behaviour per [1]. If you look closely, it is not just bullseye! Older minor releases are still available in the apt-archive repository at [2].
On occasion, you will find older versions of packages still available in the primary repository, but these are usually for distributions that are beyond the public EOL date (e.g., Ubuntu 18.04). This is because while the distribution itself has yet to be retired from the repository, no newer packages have been built. In due course, these packages will eventually be removed, and will only be available thereafter in the aforementioned apt-archive repository.
