I had a similar issue with the chain shiny and the advancement deal. My theory after multiple attempts in various ways is that the way the grass advances Is tied to the seed, aka if the step into the shiny spot doesn't advance the frame it never will and a step and turn will always advance 2. Making some frames impossible to hit.
So my theory is each frame has a set no advance via step/turn locked into it. So 1649 is probably a no advance for step but 1 for a turn. Aka when you do the turn/step on 1649 it advances you to 1650 for the turn band then 1650s step counter activates and counts your step as 1.
You can try from 1648 but my thought is that it won't matter as the turn puts you on 1649 which has a 0 frame walk advance.
My memory is fuzzy but if the step counter works in grass(128 steps = frames advanced by # of party Pokemon) then you can use the step into the patch at 1649 with it being your 128th step with 1 party member. I think this only works outta the grass though
I got it to work with a syncer at the end. I noticed that no frames advanced at 1649, so I took my step w/ sync lead on frame 1649 hoping to land on 1650, and lo and behold, no frame skip!
I had tried 1648 earlier, a turn walk in hopes of hitting 1650, and I ended up hitting 1651. So for some reason, 1650 kept getting skipped no matter what I did.
I totally agree with your theory based off my other chained shiny projects, but I never had frame skipping problems like this. I usually resolved it by advancing till one frame prior to my target, and that did it. I also had more than one party member, so to utilize the step counter I would have to either use chatot as my catcher or not use chatot at all (something I'd rather not resort too).
If anyone was interested, this is what all the trouble was about:
Modest, Download
30/22/31/30/31/30
Hidden power Fire
Meh, too lazy to do another run through just for one porygon. If I can replace HP IV to 31, I will.
This was good practice.
For me, the fun is in the journey, not the destination.
