Hugo

Power save on the wifi chip. Check it with iw dev wlan0 get power_save and I would bet money it says on.

Turning it off with iw dev wlan0 set power_save off will confirm the diagnosis within an evening, but that does not survive a reboot. Make it stick with a tiny systemd unit that runs the same command after the network is up, or if you are on NetworkManager set wifi.powersave = 2 in a file under /etc/NetworkManager/conf.d/.

The reason it looks like the Pi died is that the chip goes into a deep sleep and then fails to wake for beacons, so the router quietly ages it out of the association table. Nothing on the Pi notices because nothing on the Pi was trying to talk.

One gotcha on USB boot: if the Pi has been sitting untouched since 2019 you need to update the EEPROM first with sudo rpi-eeprom-update -a while it can still boot from the card. Do that before you wipe anything, otherwise you get a green light and no HDMI output and spend an evening confused.

One thing nobody mentions: HDMI plus a drive plus a keyboard is a very different load from the headless setup you probably intend to run. If this ends up as a headless box on a shelf, unplug the monitor and retest before buying anything. Mine only complained during setup and has been clean for a year since.