Dracula problem on statistics

I’ve started to read Dracula, but I’ve noticed that the percentages of unknown words is wrong. Every single chapter is 0% of new words. Is it a way to fix this?

What I’ve noted recently is that when you open the course page all lessons show 0% new words, if I wait a bit the statistics get updated to the actual data

Maybe the problem is just with account because I waited for almost 5 minutes and nothing happened!

It depends on your computer and on a number of lessons in a course.
Compare with any course which has just a couple of lessons.