I suspect this is actually more like: actions on Ponder answers are not covered by the "Ponder Questions" email preferences, because a "Ponder Answer" is not the same type of object as a "Ponder Question".

Specifically, PonderAnswerTransaction does not add mail tags.

This will all be mooted by T10448.

I'm just going to merge this into T10448.

If you want to "fix" this prior to that, I think it's likely to be: add getMailTags() to PonderAnswerTransaction and have it return PonderQuestionTransaction::MAILTAG_OTHER for all transactions, to cover all "Answer" actions under the "Other question activity not listed above occurs." preference.