It might have something to do with the variables for the change verbs. As you know, some verbs indicate an accidentals change and others an intentional change. Similarly other verbs indicate a change that happens quickly and others a change that happens over a longer period of time. Maybe one of these variables was relevant in the exercise.

Hola John, 

This might help-

 https://progress.lawlessspanish.com/my-languages/spanish/exercises/judge/1701/4842319

Saludos

Sorry, wrong link. This is what should help:

Using convertirse en / ponerse / hacerse / quedarse to express "to become" (Spanish verbs of change)

And maybe this one...

Using Spanish verb volverse to express to become/to turn (verbs of change)

I think at times they're interchangeable. Rather nuanced at times also. I used the wrong verb hacerse. I need to memorise them better.