Inability to Input new value before the previous value on the send assets page

[VolodLytvynenko]

introduced in #19750

Steps:

1. Enter any single number into the ETH textbox (e.g., "2").
2. Move the cursor before the entered data and input another value (e.g., "1").

Actual result:

The new value (e.g., "1") is appended after the previous value (e.g., resulting in "21"). The new value is not inserted before the previous entry.

previous.mp4

Expected result:

The new value (e.g., "1") should be inputted before the previous value (e.g., should display "12"), allowing the user to insert values in between existing entries.

cc: @vkjr