Not sure if this is a bug or removing the feature was intentional, but for the past couple weeks I haven't been able to write custom definitions for terms. All the ones I've written in the past still show up, but the option to change the definitions of terms in lessons/quizzes is gone.
The button does not show at all, in Firefox on both Android & Windows.
As of today, the option seems to reliably appear in the lessons again (previously it was gone for ALL lessons and terms, not any particular ones), but during quizzes the button still isn't there.
In your Settings > Quiz, do you have "simplify quizzing area" turned on? If so, it removes a lot of the "extra" stuff in the quizzing area, including that.