That actually makes sense to me. Duolingo is "gamefied" which is code for "works just like an addictive drug". It's designed to be habit forming by providing scheduled rewards. I'm not sure if it's actually the case but it wouldn't surprise me at all if this strongly associated the learned information with the app itself.
And what makes them similar to drugs is scheduled rewards. If you see a timer anywhere, it's drug-based design.
Duolingo is absolutely habit forming. It's got positive and negative reinforcements timed at many frequencies: days, weeks, months and years. It's got all sorts of rewards: XP, badges, gems, daily tasks, weekly leaderboards, yearly progress reports. It's got all sorts of punishments: loss of streak, permanent loss of monthly badge, loss of position in leaderboars.
The only thing that excuses all this is the fact it's trying to instill a healthy learning habit which leads to positive results for the user. Unlike the many Skinner's box simulators out there with the reward buttons wired to the player's credit card.
I really hate that part of Duolingo.