It's hard to tell exactly what the cause of loss of connection was.
It could have been that you weren't directing the antenna properly, it could have been interference and atmospheric effects.
Back when I was flying an old drone that used wifi for the connection as the Mini does, range was very variable.
Sometimes it would go further than DJI specs, sometimes less.
I'd only be concerned if it consistently fails at close range, but a one-off shouldn't be anything to worry about.
This time you've confirmed how loss of signal triggers RTH.
It's good for confidence to test and practise by switching the controller off while flying in a safe, open area to see how this works.
