Unfortunately I have found Yellow Pear to be very susceptible to wilt diseases (and pretty much any other disease too), especially late in the season when the soil gets dry. It sounds like this is what is happening to your plant.
This is assuming that what you are describing is true wilting (leaf midrib collapse) and not nutrient or fungal related problems, which harm the thin parts of the leaf before forcing the plant to kill its own leaf.
I would not blame your potting soil, but more likely the stress of the overgrown seedling in it previous pot allowed th disease to set in. It's hard to find overgrown seedlings that are completely healthy, although most outgrow the problems.