Increase the difficulty to master words
The difficulty to master new words needs to be re-adjusted back to the old formula. Currently, it is too easy to "master" them. I've already mastered a few words that haven't even been incorporated in some of the exercises.
As I understood the new method is some sort of adaptive spaced repetition. So you may see them mastered now, but in some time (depending on the algorithms and your level) the mastered will disappear and you'll have to repeat them again. Which is more effective than "learn and master it once and for all".
Even with that, I don't think the system should leave the words with mistakes as is. They should be repeated very soon - look at Memrise, after a mistake it always gives you the word a few more times in the same "watering" session.
I haven't noticed, does the strength of the word still go up even if you make a mistake? I'm just guessing the system here regarding word repetition is more or less similar to memrise, except it's more complicated as it's more than just words involved.
But perhaps the strength goes down faster than if you got it right. I mean you still saw the word and know that you got it wrong, so it makes sense that your memory of it anyway gets better. It would be best to point it out to the developers I guess.