I clearly have not accepted an answer to this question but it still shows my acceptance rate to be 100%.
One of the points explained in How does accept rate work? appears to apply here:
- The accept rate is calculated on questions that are older than 3 days.
The question in question is not old enough to be counted toward your accept rate yet - it's less than an hour old.
In addition, even if the question was 3 days old, that answer is relatively new. Another relevant point from the above linked answer:
- The accept rate is heavily cached and can take 24hrs or more to update.
So this looks like it's status-bydesign.