Removed all items; goofed about some with team order (particularly the lead position - after all, I'm not going to check all 720 permutations...); switched out the entire party for other pokemon; tried different number of pokes in the party (from 1 to full); tried depositing my entire bag into the PC; tried approaching the gym from different sides (this was something I'd already tried before, but tried it with new conditions this time); tried waiting a while after savestate-ing before the event (since I had always savestated in front of the gym to save time); tried exiting and reentering the city; tried triggering it with only a shiny in my party (did I mention my shamefully good luck this run? Found a shiny Cherubi while backtracking to buy potions in Maroon - it's adorable).
One of the few things that comes to mind that I *didn't* try was releasing any of my mons... because, well, even during a bugtesting exercise, my mind can't help but counter that /that's just cruel/. Sorry :P
So yeah, still no luck. (For a moment, I thought I had found something, since I got it to trigger properly twice in quick succession with nothing but my Zigzagoon in the party, but I'm pretty sure that was just astounding luck, as it repeatedly failed to trigger again after that :/ )
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Anyway, since I remembered it: there's a tiling error directly south of the pokecenter on north Snowcrest Path. It's easy to miss, but an eastern section of the 'path outline' is shifted 2 pixels east of where it should be, giving the outline a kink. That description may be a bit hard to follow, but if you look closely at the ground, you'll see it (it struck me instantly). (I guess it's possible that it's intentional, but if so it looks odd, and I don't think I noticed similar kinks elsewhere.)
(Speaking of backtracking to buy non-super potions, you might consider adding them to some of the later shops too (Teal, Slate, Snowcrest in beta 2). Same with regular pokeballs. Regular potions still retain a lot of value at that stage of the game, because some mons (especially unevolved ones) have to get to dangerously low hp for super potions to get their full value. (Personally, I don't like the prominent look of great/ultra balls in gen3+, only using regular pokeballs, for aesthetic's sake - though I make exception for special balls that look cool (like the dusk ball, which is what I caught Carbink with, frex)). Unless there's a mart item limit that I'm not aware of, it doesn't hurt to keep lower tier items available for longer than they normally are. :/ )