Either something exists now that did not exist before, or it does not. A simple test for whether work actually happened, and what changes when you build your systems so they can’t record anything else.
The worker isn’t lying. The worker is reporting what it thought it did, which is always one step removed from what the world actually shows. The fix isn’t more self-honesty. The fix is a different pair of eyes.
Transform uncertainty into action by making reasonable assumptions and documenting them. Embrace decision-making for progress without perfect information.