I've noticed the same.
I'm doing 100 cards a day, but several weeks ago I did 300-400 cards a day to get the repeat stack back down to zero.
The problem is that my repeat stack is consistently reset to about 400-500 cards everyday.
My success rate is from 70%-80% daily.
The problem is that by only doing 100 cards a day, my repeat stack keeps returning back to the 400-500 card range. At this rate, I haven't encountered any new cards for two weeks now. Logically, it would make sense that if my repeat stack is 70%-80% solved, that the repeat stack should gradually decrease back down to zero over the course of several days, but this is not happening.
But here's what is profoundly frustrating: extremely simple Spanish words like: yo, este, un continue to appear in my stack. I'm currently at level 13 with 1252 words in Lingvist, but it's becoming more and more evident that I'm never going to get to level 14 at this rate. It's very disappointing to look forward to new vocabulary and then be faced with words that I have mastered a long time ago.
The algorithm is definitely not programmed correctly and situations where the repeat stack continues to move back to unattainable, reasonable levels will just make people like me move on to something different.