A cynical reason may explain why the PIS do not show correct details on special or diverted services, is that the person's (guard or driver) union wants an extra payment for operating the system in this situation and the TOCs have said go away to the union. Therefore only the proactive member's of rail staff say sod the union, it is more important that the correct information is displayed and try to set the PIS correctly, but only a minority of staff think like that.
The pis functionality is part of the training course for the crew on a 701, so this is not the case here.