That's the odd thing. I replaced all the railings with a base game option (Living Stairs Rail) in order to get rid of the preset flag banner railing that came with the DLC bundle. (Keep in mind that I did this because I'm turning the venue into a Festival Lot.) It is only that set of stairs that is causing the bug. All the other sims don't bother using the second pair of stairs on the other side that lead into a tree. They can also travel back and forth between the courtyard area and the Archery Range with no issues. It's just that set of stairs, for some reason, if they are traveling from the bathrooms to the Archery Range.
I've tested it recently and discovered that if a Sim does get stuck in that loop, either a NPC or a Player-Controled Sim, that nobody else can use that set of stairs despite that it is also 2 squares long.
I still don't know what causes it, too. The first time I sent my Sims to test out that specific set of stairs, they went up and down just fine. And then an NPC got stuck in the loop. The next several times, I was in a group, and one of the Sims I had control over, who was not the active Sim, got stuck in that loop too while trying to talk to another Sim (according to their action queue).