So I did it on the rom no issues there, though it was a similar party to my retail cart attemp.
A glimmer of good news(kinda), I just right now, I remembered a time in Diamond where I had a similar problem, where I would my target frame(IV's and nature would be correct) but the encounter slot was for the pokemon on the frame before that of my target(I kept getting a calm parichusu instead of a shellos).
After like 10 times I couldn't figure it out and I ended up switching locations. So my next suggestion is to find a different cave(if thats even possible) to RNG it in.
I'll check on if there are multiple caves, or levels, and get back to you(along with what cave I did mine in).
If that doesn't work, I'll try it with Irwin calls(I've never done it that way). and get back to you if that causes me any grief.