"When you call SetCtrlVal on a text box, SetCtrlVal appends value to the contents of the text box. Use ResetTextBox to replace the contents of the text box with value."
Everytime I use SetCtrlVal() it replaces the contents of the text box rather than appending as the help suggests.
You have correctly interpreted the help. It may happen that the new text entered moves old contents of the text box out of the viewable area, so that it seems that old text has been replaced. Try enlarging the text box or retrieving its value in a string and look at the string.