Why does my list suddenly reorder itself?

That's intended — and exactly what makes pon better for you. Two things move the order:

• pon keeps learning from your check-offs how you move through the store and adjusts the sorting step by step; after a few trips it settles and stays calm.
• The section of a known store automatically moves to the top as soon as you're there, so you immediately see the right things.

You get the most out of it by simply letting pon learn along — then over time you just work down the list from top to bottom in the store, without searching. A fixed alphabetical order is possible, but it throws away exactly this advantage.