LE just posted their own (excellent!) incident report of this on the mozilla bugtracker, including the discovery timeline, analysis of the bug, and follow-up steps: https://bugzilla.mozilla.org/show_bug.cgi?id=1619047#c1

The original bug report, which was initially diagnosed as only affecting the error messages, not the actual CAA re-checking: https://community.letsencrypt.org/t/rechecking-caa-fails-wit...

Brief discussion on revocation exemption requests: https://bugzilla.mozilla.org/show_bug.cgi?id=1619179

Tomorrow will tell if granting a revocation exemption might have been a good idea in hindsight.