I found this easter egg originally on my alt account. Note, this account finished the quest before feeding the foxes. I went back and fed them later and after waiting hours irl I stumbled across the fox. It might be a night time reset event, since I waited hours in game and never could get the fox to spawn. I also went to the bottom of the mountain and walked back up to see if it was a proximity event, but the fox still didn't spawn.
It seems miHoYo slipped this easter egg in front of us without many of us knowing. After all, the only way you can find it is if you actually read what a quest is saying when most gamers, myself included, just press "skip" or click through events.
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