It's not incorrect, it just wasn't added as an alternative yet. Please go ahead and use the report button anyway in the future, even if you have doubts.
"have transformed" was added for a hint in the sentence "Ik ben veranderd in een schildpad." for which the best translation is "I have transformed into a turtle."
The hints added in one place are automatically added for the word(s) everywhere else in the course. So sometimes even if a translation shows up in a hint, it might not be in the alternatives.