I do not remember there being any issue in the past, but more recently I noticed that the horde attack does not proceed and is not continuous when the party is on top of the platform which one must climb up a ladder and over to reach the marketplace.
I can hear the horde attack starting, but sometimes they do not even come into sight, much less attack. To trigger their approach, I have to jump off the platform, then climb back up to defend from there. I have to do this several times to in order to trigger all of the horde to approach. In the attached image, they are just standing there and not climbing up to attack.