Having done a bit of checking on Avantix all the London Midland Only fares I checked, both Off Peak and Super Off Peak, do not permit break of journey and so it is actually very possible that barriers will have been programmed to reject tickets and that barrier staff will have been made aware to watch out for it too.
Given Virgin also restrict break of journey on some/many of their fares now I'd expect staff to be even more aware. As for it not being enforceable to restrict break of journey on the return portion, it's in the fare manual therefore enforceable (whether ethically right or not). I agree that many staff aren't aware of break of journey rules and often don't object but certainly in SWT land we get issues with barrier staff not believing that a break is allowed in either direction (Winchester, Guildford, Basingstoke and Southampton all spring to mind immediately). Also I have had issues breaking journeys in the past at Watford J, Milton Keynes, Coventry, Birmingham NS, Wolverhampton and Crewe where I've had to argue with staff (sometimes barrier staff and sometimes an RPI blockade) which are all in London Midland area so to say you would never expect to be challenged may be your view but certainly isn't the case at MANY stations (Stevenage, Bristol TM, Bristol Parkway, Salisbury, Colchester to name a few more that spring to mind). Things have gotten better recently and problems are fewer and further between but certainly not never an issue and often depends on the person you encounter at the barrier.
I do agree that London Midland certainly have in the past encouraged starting short on some tickets so contradict themselves which makes this whole situation even more of a grey area. Given they are on this forum and normally very open and up front about solving issues maybe they can clarify what their current policy is on this as why add that restriction to their tickets if they don't care about it?