Knowing the area well, I can confirm that lane 1 if a dedicated “off lane” going to the roundabout for the A34/A41, with lane 2 & 3 carrying on for the M40(S).
Good question! We had many arguments about this sort of thing.
There are disadvantages to a “keep right” command for traffic continuing on the motorway. It stops Waze from displaying the distance to the exit that people eventually want to take, and the junction number for that exit. Also, people get tired of continual "keep right"s when they are staying on the same road.
The situations where the consensus favours “keep right” commands when staying on the same road are now documented in the Wazeopedia. In other situations, "keep right"s should not normally be used.
I think this is one of the cases where Waze should keep quiet; only one of the original lanes is dropped (even though it has split into two by the time it actually departs), and it is easy to rejoin from the roundabout if you do go off by mistake.