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Drift
It worked when you launched, and it does not now, and nothing obviously changed.
The model was updated, or a dependency was, or a policy was, or the data coming in gradually stopped resembling the data it was tested on. None of these announce themselves. Drift is why a launch is the beginning of the risk rather than the end of the project.
Why it matters
Whatever made it safe to ship was true of a moment, not forever.