Please put the statement
Disable auto return
into the InputScript.
If the processing of an InputScript ends with a new transaction, GuiXT
interprets the next F15 key as a return to the original transaction where the InputScript started. This makes sense in many cases, but not in a case
where the InputScript itself already realizes a "backwards"
functionality, as it is in your case. With "Disable auto return" the standard F15
remains valid after the InputScript processing.