I have to correct myself, this glitch only happens when I try to immediately type the word again, without first hitting backspace or enter (if I do that, there's no problem). Also, it doesn't happen every single time.
I used my other IOS device and I was able to replicate this problem: I created a flashcard with the word νιπτήρας (bathroom sink), then if I type in Greek with a mistake, the app shows me the correct spelling (with mistakes in red) and when I try to type the answer again (without hitting backspace or enter), the letter σ sometimes appears at the beginning of what I am typing, even though I did not type it.
In general it's not such a big problem, so no worries, I can live with that, it's just that sometimes when I don't pay attention and don't remove this σ at the beginning, it of course doesn't accept my answer as correct and then I get to repeat this particular flashcard more often even though I don't need to