There is a bug in the upgrade scripts somewhere. I recently upgraded from 3.0 to 3.1 and to 3.2. The captcha method reset itself to an image but it was set to Q&A before.
As a result there are ~150k spam posts in my forum now.

The old version must have been at least 3.0.6 as the Q&A captcha did not use a plugin, it's built into phpBB. All the questions and answers were left intact.