Although my system seems fully up-to-date, pureos-upgrade prevents the upgrade and indicates “Your system appears stuck on old package versions. Please contact support for assistance”. All other boxes are ticked.
We wasted quite some time looking for issues on the PureOS infra side just because we initially assumed the problem was infinitely more likely to be there than in apt. So yeah, we were just as surprised
Turned out that it was being triggered by certain circumstances that are present in PureOS by default and not present in most Debian installations, but that it was an apt issue after all.