Dead.letter for root after system boot / Could not resolve host: github.com

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22

seem it is triggered for root upon restart (@reboot)

23

for me it seems obviously that /etc/cron.d is checked by root and the pihole file is processed doing update check activities.
I dont know where the 30s delay you mentioned is coded for pihole, if so, then the @reboot line in /etc/cron.d/pihole is not needed or should also be delayed to give NW a startup