> I'd like to understand how I failed.

I think the main problem is that you don't set 'PORTS_PRIVSEP = Yes'
in your /etc/mk.conf, which allowed your build to download stuff from
the internet. It's a bit of a pain to set up, but there are some hints
and breadcrumbs in bsd.port.mk(5).

Since bulk builds are using privsep, it is really important that you
make sure that ports don't require internet access to build.

> I built it (with QT5 & 6 after your message) and installed it and it worked.
> As a previous version was already installed, I guess I already had all the
> required dependendies.
> Next time, I'll build it from scratch on a pristine OS.

You must look through configure and build logs and check that every
dependency found is registered as such, that nothing is fetched from the
internet and for dependencies that aren't found you need to see if they
could be accidentally be picked up.

And to state the obvious: another good way to avoid such issues is to
ask for review on the list rather than committing without ok.