It must not be part of a “named trail”. Apparently that’s the key factor according to @NianticAaron , even though there’s no such requirement listed in the criteria clarifications and there are literally two examples in there where the markers are not part of named trails but are acceptable anyway.
By the way, Aaron, another one of my submitted sticker markers was approved today. This one was in Niantic Voting. So both your AI and your employees are approving these sticker markers. Maybe that means they’re not quite as ineligible as you think they are?