When track changes disabled the backspace is OK:
- User typed two lines (1) & (2)
- At the beginning of the line (2) user presses the backspace key, it successfully appended the line (2) with line (1) (where there was a space available). 
When track changes enabled the backspace does not work as expected:
- User typed two lines (1) & (2)
- At the beginning of the line (2) user presses the backspace key, it removes the end of the character in line (1).