Same here. It started a few days before Christmas and it is still the same today. I expected they would solve the problem right after the new year, however, what I found was 1000+ words in the repeated stack after a short vacation.
I spent 5+ hours just to clear all 1000+ words and got to do 20 new words, and the other day 400+ more were back.
I still have a few days off for my vacation so I could just waste the whole day on Lingvist right now, but it would be difficult to keep up once I go back to work.
I do think the algorithm problem is more serious and more important than monetization at the moment. No one will pay for a broken product.