That's by the way a tricky one. If you are a witness on a mainnet, your witness updates are tried to be reproduced on a testnet, so one becomes a testnet witness without knowledge and consent, and obviously starts missing blocks because there's no node to sign them.
Indeed, I noticed that too while updating to 20.12 and switching signing keys on the mainnet. I missed blocks on the testnet again until I reset the testnet signing key to produce there again.
That's by the way a tricky one. If you are a witness on a mainnet, your witness updates are tried to be reproduced on a testnet, so one becomes a testnet witness without knowledge and consent, and obviously starts missing blocks because there's no node to sign them.
Indeed, I noticed that too while updating to 20.12 and switching signing keys on the mainnet. I missed blocks on the testnet again until I reset the testnet signing key to produce there again.