Just started a new learning session (web app) with cards count (at the bottom of the page) equal to 0. Progress page showed 120+ cards in the repeat stack. After competing 100 cards (some of which I failed and had to relearn) I got 0 card in the stack (on progress page). Though Lingvist keeps throwing in "repeat" cards (that is expected, according to the initial estimation, but is strange considering current values). Looks like the math is pretty off here even for a perfect session,
I can confirm that sometimes there is something wrong with the math.
Today I did round about 30 cards. My repeat stack was empty, but Lingvist kept on challenging me with already learned cards.
Then I signed out, closed my brother (with delte cookies on close option on) and signed in again and then Lingvist told me, that I did 0 cards today, even though I just did round about 30.
Then I did 70 cards, logged out, logged in and the counter for todays cards was again 0???
The timewindow in wich I learned today was between 5 and 6 pm GMT+1 (+2 at the moment because of daylight saving time).
Interesting. After you signed in and started a fresh session with 0 words, was there a sudden spike in word number after a few words repeated? It's very typical for Android app to show 0 after just after startup, but then it catches up. I don't really often sign in/out on desktop, but I wander it the behavior is similar to mobile.
There are lots of broken things and technically induces peculiarities on stats page, that should not be visible to end user, but the issue you are describing, looks different to issue I've described previously, at least from where I stand.