Withdrawals will always come out of your "Core Savings" bucket before any of your other custom buckets so that you don't accidentally lose your progress toward your savings goals.
You can change the order of what buckets withdrawals come out of first by following these steps:
Tap the Money tab on the bottom.
In the Buckets section, tap the "Tools" button.
Select "Reorder" in the popup window.
For each bucket, you'll see a multi-dot icon to the right. Tap and hold the icon to drag and drop your buckets in the order you prefer.