If this can be reproduced with pkgin 23.8.1 and against a consistent package repository then I'd certainly be interested in a bug report for it.
And I think I found what the issue is.
Restored from backup. As you described, I upgraded pkgin and pkg_install first, with „pkg_add -U“. Now „pkgin upgrade“ shows this conflict:
py311-cairo-1.24.0 (to be installed) conflicts with installed package py310-cairo-1.23.0nb1.
The older pkgin did not warn about this (or at least I don’t remember seeing this warning), so I assume that’s what broke during the upgrade. For now, I manually removed py310-cairo and hope for the best.