Completing unit on iPhone doesn't "stick"
I finished the Spanish conjunctions unit twice on my iPhone (on a call, so was without connection via wifi/cell signal) and right after it said congratulations you finished, it took me to my home screen showing I still had to finish that unit.
Got off the call, went to my laptop and did the unit test out, and almost immediately it sync'd to my phone that the unit was complete.
So, is this a bug or do I have to be connected to the internet for progress to save on the app?
It sounds like you need to be connected to the internet. If I close my laptop screen mid-lesson, I can answer everything and it will just not count towards anything. Not sure if that sounds quite similar to what you explained though. I would just try to not do anything that could disrupt Duolingo.