After recent maintenance applied to Endevor, users have lost the ability to select multiple entries from selection lists other than the element selection list.
It happens in options from the DISPLAY menu like display system, display type, display processor group and so on.
Endevor displays the first selected entry but, when user hits PF3, Endevor returns to the selecion list instead of displaying the next entry. Moreover, the 's' selection characters have been removed from the list.
This behavior was introduced by fix LU11840 (in release 19.0) or LU14173 (in release 18.1)
Apply the fix that corresponds to your Endevor release