At 01:20 UTC on Saturday, Jan 29th, 2022, while remediating the TLS Using ALPN incident, we discovered two certificates for which we were not serving OCSP data. Further investigation revealed a total of 132 certificates for which we had never served an OCSP response.
All affected certificates now have proper OCSP responses being served, and the bug which gave rise to this (very rare) circumstance has been fixed. You can read the full details of the root cause and remediation in the report on Bugzilla.