The correct, consistent and language-specific formatting of "Wunschbetrag" is in PR 271, but not integrated in EN banners and current ipad banner. It will be implemented gradually in the next tests.

The requirement of "amount does not go away and is editable" needs some UX input IMHO. This is a mobile-specific behavior and as such I think it was introduced intentionally. I find it more convenient to type an amount again on mobile than having to painstakingly move my cursor (Apple has made that even harder with iOS 13) or press backspace multiple times. What do you think, @Jan_Dittrich ?

It seems that keeping the value in the field is the standard, thus I would suggest also keeping the numbers. (or in general: Do whatever the standard input-for-numbers behavior on the device is)
The larger problem might be that we attach a €-Character directly to the input, which leads to confusing appearances and disappearances of the € and makes hitting the right spot with your cursor harder (since "after the number" needs to be in the small spot "before the €"). Biggest gain for the field is getting rid of overwriting the value and showing the € via CSS.