> > child process exited with exit code 1
> > initdb: removing contents of data directory "/var/postgresql/data"

Known fallout from a change in libc.so.100.3:
https://marc.info/?l=openbsd-misc&m=172434747515381&w=2

> on some other host I have a happily running 16.2. Haven't yet spotted the difference.

It will be running on libc.so.100.2 or earlier