Selected text disappearing from notes

There is no way to recover that now. If you had used shake to undo all would have returned immediately.

Probably you are not aware of shake to undo/Redo and both. Please see the pic below which will help you for the next time if that happens.

Have a look at this article for ways in addition to shake to undo: Adjust how iPhone responds to your touch - Apple Support

"To undo text edits, swipe left with three fingers."

By the way, this non-Apple article outlines even more features: https://www.wikihow.com/Undo-in-Notes