It looks like what appears after "Enter your selection:" is a command prompt. So by the time you type "8<enter>", you're no longer in promon; it is being interpreted as a shell command and korn shell tells you that it isn't a valid command.
So something or someone is killing promon/proshut, or it is crashing after showing you the options, without throwing any errors.
Good point. Type "which promon". If the output isn't $DLC/bin/promon then you have some script or alias that is intercepting that command and doing something other than just running the command you intend.
If the output is $DLC/bin/promon then check its contents and ensure that no one has altered it since OpenEdge was installed.
This looks to me like an issue with your environment setup, not an OpenEdge bug.