Leave command: I noticed that the GuiXT script for a screen is not processed when I leave the InputScript with

Screen ...
  Leave

Is this a bug in GuiXT?

No, it is designed like this in order to allow the InputScript to display a certain screen layout at the end. For example:

Screen ....
  del F[...]
  del F[...]
  Pushbutton ...
  Leave

If you want the GuiXT script to become active for this screen, you have the following options:

(a)
Terminate the InputScript processing before processing this last Screen command.

(b)
Execute an "Enter" command for the last screen:

Screen ...
  Enter
  Leave

If you don't want any SAP application processing performed for the screen you may use the function code "?" which does nothing, apart from redisplaying the screen:

Screen ...
  Enter "?"
  Leave

(c)
Include the GuiXT script for this screen, e.g.

Screen ...
  Include "sapmv45A.E4001.txt."
  Leave

Option (a) is normally the best.