It's absolutely valid. There are no evening peak restrictions on a Milton Keynes to Coventry Off-Peak Single/Return routed "via Rugby". The evening peak restrictions under restriction code 9I would only apply if you boarded at Euston, which is why the electronic data restrictions are set up to allow you to use such a ticket from Milton Keynes in the evening peak. As
@jfollows has quoted, the restrictions for Milton Keynes are more specifically defined as only applying in the morning peak.
The reason for this disparity in restrictions is that, under the terms of their agreement with the DfT, Avanti aren't allowed to impose evening peak restrictions on regulated Off-Peak tickets for journeys outside of the 'London area'. For the purposes of the WCML; that is defined as meaning stations between London and Watford Junction (inclusive) - hence Milton Keynes cannot have evening peak restrictions.
As above, this isn't a new situation or any sort of loophole; I'd expect barrier and on-train staff to be familiar with the difference between London and Milton Keynes tickets.