Subjectively, the presence of food or drink implies a concern for your physiological wellbeing. Since it was not a defined contractual element of the exchange, it also represents a bonus, an above-and-beyond.

It would be interesting to have tested this during the time of fidget spinners or some other tchotchke, to remove the food element but that aside, I think a weakly confirming result would be confirming what we expect: signs of care and emotional engagement beyond the immediately neccessary improve mood, and so improve some outcomes.

There is a saying that positive reinforcement beats negative reinforcement but it comes with an implicit corollary: negative reinforcement beats nothing: That negative pupil teacher interaction reports exist with higher scores should not be surprising, because it is possible SOME people achieved higher scores, despite hating (and negatively rating) their teacher.