Is it possible someone created a simplified screen that blocks editing of these fields? 

No.  We have no configured screen for either Routing Maintenance or Routing Manitenance (Rate-Based) and I can also never edit the end-date, even in Terminal mode

When you say you can never edit the end date are you talking about in the first screen or the second?  You cannot change it in the first screen as it is part of the key to the file.  Same as you cannot change the start date in the first screen, if you do you actually create a new record.  That is why they are both in the second frame.  If you change them in here it updates the key to the record to reflect the new dates

This is an old issue for me.  I opened this a long time ago.
Where my struggle was that I could not force the user (batch in this case) out of the screen because the data was not correct.
I do not have access to the code (encrypted), so I setup a procedural trigger to validate the date first, then apply.
The validate worked, but the program continued.  I was told that there was no way to abort a program from a procedural trigger.
I have had other instances where this has come up.  I wish there was a way to add validation to QAD maintenance field entries when the code is not available to be modified.

Does anyone have suggestions?