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Wouldn't that mean it's the other person's channels that are the culprit?
Usually, yes. If you are the sender, there's a failure, and you have enough liquidity to SN's node and your sending wallet is working well, it usually indicates the receiver's wallet had a problem receiving the zap.
We have a PR for automatic retries pending review which should drastically reduce your exposure to these intermittent failures that aren't your fault.