That’s happened to me a few times.
Your problem is in the second pop. When you split your index and thumb, make sure that the triangle formed is not corked forwards or backwards. That is what will give you the dismount knot.
Imagine dismounting a GT from the wrong side. That’s what you’ll get; but more complicated
[[[[When I learned it, I though you had to drag a string on your thumb from the first pop to the second to open up that triangle, but it turns out that you just have to drop that and grab the closest string to your thumb, mid-pop. ]]]]
Try doing what you normally do and examine the crux of the triangle on your left (near the middle of your hands). Remember that. Then do it again but try and slow down the pop as much as is comfortable to see exactly what strings you are spreading, then let the yoyo fall in front of all the strings instead of through the triangle (examine that if you want). Then, manually hop the yoyo into the triangle as you normally would and examine that triangle crux again. If you don’t find a difference between the your way and mine, then I don’t know what else to say.
Let me know how that works out for you.