O/D on Nissan 2017 Quest and engine rev increase

My 2017 Nissan Quest has an O/D overdrive on the stick shift.. It is always on, the
van runs fine.. or so it would seem, but when you take the overdrive off, a light
comes on telling you its off, and the engine then seems to not shift gears, it  goes
into low and high revs, it only stops when I turn it back on..
From what I understand the O/D should be off until needed at high speed... Now this
does have the CTV transmission, not sure if that makes a difference?
The question is, is this normal? should I just drive with it on? seems to run just fine
with it on and I get great gas mileage, but dont know if its hurting the engine? I cant
find any information on line about this, there are a few people complaining about it,
but no one has answered as if this is an issue or is this just some sort of extra low
gear for hauling stuff? if anyone knows of what causes this or if this is just normal
operation of the CTV transmission? thanks.