It's not working on my 6 plus either. I contacted support and they said that spotlight search is not compatible with the latest version of the app. I know it was working for me at some point so I don't know if they removed it or not. Do you have the latest app version (1.4.3) on each device?
I looked more on my iPhone 6 and last spotlight search for outlook is sep 18. So an update I did for outlook broke it. Since I just setup this new iPad mini it had the latest broken outlook from the start therefore nothing shows up on it. I have sent this info to support but you should also - the more the better. Updated outlook this morning and did not fix either.