Google posted in the data anomalies page for Search Console under the Accelerated Mobile Pages Report that they updated the report based on updating their "scanning process" for errors. It reads:
You might see a fluctuation in your AMP data, due to a change in our scanning process.
I checked my report and I do see a drop in errors after the update line on September 19, 2016. The confusing part, this comes right around the same time where AMP rolled out to core mobile search and the AMP traffic spike which has no explanation.
Here is my chart:
I asked John Mueller about this and he said on Twitter:
It's an internal change re which AMPs to include in that report; it doesn't affect indexing/ranking. Confusing timing, sorry :-/
Yes, confusing times.
Forum discussion at Twitter.