Hi there, @PremierRef2019.
QuickBooks Desktop automatically resorts itself after you select the transactions to reconcile. I agree with @FishingForAnswers about clicking the Amount column to fix the auto-sort.
Once you click the Amount column, all selected transactions will be sorted at the top. Then, you can freely select other transactions by their amounts without the transaction list randomly moving every time you select one.

You can also consider closing your books to avoid unwanted changes at the end of every fiscal year.
Let me know if you have further concerns with QuickBooks Desktop. We're always ready to help.