Hi there,
I not sure I got your situation right or not but this does sound very much like what happening with my mum's car back in Thailand. We have a 1996 Rover 623 GSI automatic. The car have been driven for 102000km without ever using cruise control once due to traffic condition in Bangkok. Anyhow almost 2 years ago we had a long drive to the South of Thailand and we finally have chance to use the cruise control.
From my experience with that car (Rover) at 100km/h in overdrive mode the car is running on 3rd gear. Without overdrive it is running on 4th gear. I drove with overdrive on most of the time and everytime I engaged cruise control the overdrive light gone off adn as soon as I tapped the brake (disengaged) the light came back on. This happen even when I was driving at 80km/h.
So from that I assumed that it is normal for overdrive to drop out when engaged in cruise control. Driving in overdrive does consume slightly more fuel and, according to Rover, the cruise control is design to work to save fuel and maximize performance (eg: right RPM at certain speed does help to save fuel). So I wouldn't worry too much about that if I were you. But again, your car is a Japanese car and the Rover is British so there maybe something different in the design and function.
Hope that info helps
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