If Amazon insisted on shipping an identical product out of a west coast fulfillment center to an east coast customer for which they had a fulfillment center 20 miles away holding the same SKU, they would not be able to offer the kind of service that they do.
“Comingling” is the fundamental basis for them to leverage their scale to provide unmatched service. It’s essential to their business model precisely because how enormously valuable it is to Amazon and to its customers, and the way it gets better with scale, despite the concomitant lesser inconveniences.
Fundamentally, Amazon still knows exactly which seller provided each buyer with each item, despite there being only a single shared product page. So they already have all the data, and of course they are using it internally to track per-seller review scores, refund rates, etc.