Sudden (and Erroneous) Invalid Site Key Error

I had a working multisite using ANR on a subsite-by-subsite basis, with the same v2 checkbox reCaptcha keys for each subsite.* Captchas were working correctly everywhere we wanted them, on the parent site and all subdomains. Everything was good for at least a month.

Yesterday, reCaptcha started throwing an “Invalid Site Key” error. This created a lockout situation for the WP dashboards of the network sites.

I confirmed at Google that my keys were still valid for the domain and subdomains. I confirmed via phpMyAdmin that the sites were still using the correct keys.

I disabled ANR (via FTP folder renaming) to get into WP admin. Subsequently re-enabling ANR (reverting the folder name and enabling the plugin via the WP dashboard) seems to have things fixed on the production network (nerdnite.com and subdomains). I did not need to update ANR’s settings.

Questions:

Do we know what went wrong?
Is there anything we can do to prevent it from breaking again?
Is a lockout the expected (and desired) result for the “Invalid site key” error?

(I note that when no keys have been entered in ANR’s settings, reCaptcha doesn’t render; to me this seems preferable to a lockout, but I’m no expert.)

I have left my staging setup in the broken state for the sake of troubleshooting. (Yes, the sudden error happened on both the production and staging environments.) Here’s a sample page with the broken reCaptcha on a Contact Form 7 form: https://sf.nnalphastaging.wpengine.com/contact/

Please excuse non-HTTPS warning and the debugging output on the staging server.

Many thanks!

* Note: When migrating and updating this network, I could not get the anr_same_settings_for_all_sites filter to work. Maybe this was because I included it in a custom plugin instead of adding it to functions.php for each of the five themes in use on the network?

The page I need help with: [log in to see the link]