GnuPG: private key suddenly missing?

After updating my workstation, I noticed that keychain reported that it could not load one of the GnuPG keys I passed it on.

 * keychain 2.8.1 ~
 * Found existing ssh-agent: 2167
 * Found existing gpg-agent: 2194
 * Warning: can't find 0xB7BD4B0DE76AC6A4; skipping
 * Known ssh key: /home/swift/.ssh/id_dsa
 * Known ssh key: /home/swift/.ssh/id_ed25519
 * Known gpg key: 0x22899E947878B0CE

I did not modify my key store at all, so what happened?

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