This problem has, indeed, been going for decades. Possibly not helped by GWR putting up misleading signs saying that 'Off Peak' Tickets are only valid at Off Peak times, ie. implying that the ticket type that governs completely the validity of the ticket from Paddington. Which is the 'rule' that clearly most staff are trained to apply even though the rule is actually a very different one ie. it is the validity that is actually on the ticket that applies, which varies by origin and destination, and this applies to all tickets that do not necessarily have London Paddington as origin or destination but are valid through it.
Once all fares are controlled by GBR, one imagines that they will simply scrap all this and simply apply the 'rule' that the staff seem to think applies curently. Then try and ride out the negative publicity from it. The approach only arises because the validity of some tickets is controlled by others (ie. not GWR). Once it is all one big happy family, it is likely that they will want to switch to ticket type as the determinant of validity....