Actually, given I trained a pony to do just that (let me jump off and do something without tying him up) at the age of 14 I will give you that one if the character concerned is an competent rider and the horse is well trained.

It’s called ground tying and it’s very much a thing. Now, if they have never handled the horse before, it’s semi-wild, or they just got on a horse for the first time that day then yeah it’ll be side-eyed.

But it is something you can teach a horse to do safely and which I honestly encourage because it is very useful.