This is one of the serious problems with FBA and serious problems with FBA's co-mingled inventory.
When I was selling inventory on Amazon, I actually paid the extra cash to have my inventory placed in a separate location so that I didn't have to deal with other seller's stock/quality issues. It was worth the additional cost (from my point of view) and it worked quite well in ensuring that Amazon's customers who sourced from my stock got the correct product and that I didn't have to deal with angry customers or unexplained returns.
When I was selling inventory on Amazon, I actually paid the extra cash to have my inventory placed in a separate location so that I didn't have to deal with other seller's stock/quality issues. It was worth the additional cost (from my point of view) and it worked quite well in ensuring that Amazon's customers who sourced from my stock got the correct product and that I didn't have to deal with angry customers or unexplained returns.