Hi @stanwise,
I'm happy to hear the problem hasn't resurfaced for you.
This was related to a new approach to calculating an existing rate limit that was introduced in master with 71f8ae. We were able to cross reference the information you provided with when this feature was enabled in production and identified that it interacted poorly with certain issuance patterns.
Since this code was feature-flag gated per our usual practice we disabled the feature flag as a configuration change which is why you aren't able to see a fix in the changelog. As you observed this was done on Aug 10th 
See this API announcement post for more.
At this point I believe we intend to abandon the approach in master and will revisit with a more performant solution involving a database migration in the future when we have the resources on both the dev and ops side available.
Hope that helps clarify!