Thank you both. I would have thought mallards would have been one of the most complex taxa that iNaturalist deals with in terms of # of listings of establishment (again, mallards = 1,469 listings), but it turns out that western honeybees (16,618 listings) and Asian lady beetles (16,650 listings) are more complex by an order of magnitude.
Thank you @thomaseverest for separately flagging the creator of the custom place in question – agreed that I should have tried a flag first. Hopefully it can be resolved soon. If such a rule could be created, it seems reasonable to require created places to be within the boundaries of parent places to avoid errors like this.