Engineer/Constructor do not have sentry flag cleared after build completion.
If Engineer or Constructor build unit from sentry mode, they stay in sentry after build is completed and are skipped with next/prev unit action. Build needs to clear this flag. I think the answer is in /game/data/units/vehicle.cpp:proceedBuilding(). Something like this seems to work:
--- ../maxr-0.2.9.orig/./src/game/data/units/vehicle.cpp 2016-10-08 22:49:18.207230981 -0700 +++ ./src/game/data/units/vehicle.cpp 2016-10-16 12:42:02.526753540 -0700 @@ -328,6 +328,7 @@ bool cVehicle::proceedBuilding (cServer& const cMap& map = *server.Map; getOwner()->addTurnReportUnit (getBuildingType()); + setSentryActive(false); // handle pathbuilding // here the new building is added (if possible) and