TLDR: Green was freezing daily, so I held the button for 6 seconds to do a graceful shutdown. Came back hours later to find it fully factory reset.
What happened today: My HA has been a bit unstable lately with daily soft-locks/freezes of unknown cause (the logs just contained some timeout errors from the Tuya integration, but that's another topic). What I did previously when that happened, was just cutting the power, waiting a few seconds and then plugging it back in. Today I noticed/remembered that there is a physical button on the Home Assistant Green, so I looked up how it works exactly and tried a graceful shutdown by pressing the button for 6 seconds (according to https://support.nabucasa.com/hc/en-us/articles/25207565119133-Shutting-down-Home-Assistant-Green ). Then something came up and I left for a couple of hours. After trying to log into the HA dashboard this evening, I was greeted by the welcome screen. My HA was fully reset...
A factory reset should have been only performed when cutting power and then having the button already pressed when reconnecting the plug according to https://support.nabucasa.com/hc/en-us/articles/25207565119133-Shutting-down-Home-Assistant-Green .
However, I only pressed the button once when it was on to shut it down, and at no point was the power disconnected...
(well, and of course I have no backup, but that's entirely on me)
Do you have any idea why this happened? Besides, even if this hadn't happened, I really think for a full factory reset, 6 seconds is way too short.