Builds with SSH user keys no longer run

We’ve been using SSH keys defined via the project “SSH permissions” tab for over a year and they’ve been working fine.

Just over a day ago, all builds are now failing:

Your build was not run - reason code (no-ssh-key).
We could not test this push, because the Github repo is missing a Circle SSH key. You can fix it by having an admin click the Add SSH Key button.
Project XXX isn’t configured with a deploy key or a github user, so we may not be able to test all pushes.

This has started to affect all our repos and there aren’t any recent changes to our Github organisation or our CircleCI configuration that would explain it.

We’ve had automation to populate these SSH keys:

https://circleci.com/api/v1.1/project/github/{repo}/ssh-key

and rerunning that or manually deleting and adding the key again also fails.

Looks like something changed in CircleCI, since it’s not even attempting to start the build. Any suggestions?