You missed the point. Journey planners are required to use data supplied by the industry. In this case the data says the ticket is not valid on Mondays. So the journey planner is correct. Journey planners are not supposed to change rsp data.
I think you've missed the point. It's the data supplied to the journey planner which is incorrect. If the date shown on the Super Off-Peak Day Return ticket is a Sunday, the ticket is valid for travel up to 04:29, the following day. There is nothing in the terms and conditions of that ticket which make an exception when the following day is a Monday.
The question is whether the data supplied to journey planners is incorrect through a lazy (not valid on Mon-Fri) interpretation of the actual ticket restriction, or a deliberately more restrictive implementation, in order to force the purchase of the more expensive ticket.
The way restriction code BT is implemented by journey planners for Off-Peak Return (SVR) tickets is another case in point. The actual, published, restriction sates that the ticket is not valid for outward or return travel by any train timed to depart before 04:30, also that where travel is to/from or via Luton, Luton Airport Parkway or London, restriction codes CI and CJ also apply.
So, restriction codes CI and CJ should
not apply to such SVR tickets, where travel is
not to/from or via Luton or London, e.g. from Tamworth to Loughborough, using the SVR, which has a BT restriction.
But check out a journey planner for a return journey from Tamworth to Loughborough with early morning outward itineraries. Even though the trains don't depart before 04:30 and it's valid for travel, the SVR (£21.60) isn't offered. Instead, you get the more expensive option of single tickets (£45.30) because the journey planner is using incorrect, unpublished restrictions, which lazily (or intentionally?) implement restriction codes CI and CJ more restrictively, to apply to
all travel on trains arriving at Bedford or London in the morning peak, not just travel to/from or via Luton or London on them. The early morning itineraries from Tamworth to Loughborough involve use of such trains between Derby and Loughborough.
So the punter shells out the £45+ to purchase the singles which they believe are the cheapest option, as that's what the journey planner tells them, yet it's double the price of the ticket they should be being offered. In other retail environments, this would be called price gouging.