Okay, must have been confirmation bias. I jumped to conclusions.
Sorry for false testimony.
I looked at replays of YI matches, then just watched the stage in two eight-minute matches I set up with two controllers where the chars just walked around.
The plats show up in intervals of something like 11 seconds... except it randomly varies around 11 seconds to sometimes 12.5 seconds, and I think as low as 9 seconds. EDIT: In any case, it's never actually a clean integer value in seconds, because the digit on the timer was *always* tough to call precisely. Then, I *think* periodically, but I was relying on memory just for these tests, it tosses in about a 15 second delay before running in this manner again. My attention drooped sometimes and I think there are even 25 second gaps.
I thought I could influence or predict which side it pops up on, during these tests....
Well, while I'm off studying the level (with charts now!), at least you can say: Consider the appearance of the Support Ghost when edgeguarding.