Amazon keeps different sellers inventory in different bins, which are tracked.
I agree that some sellers are avoiding it, but I know several big sellers that are all commingled.
Most of my business was merchant fulfilled, but I still sold over six figures worth FBA last year. Never had an issue from commingled.
If it's very accurate, and keeps things separated, you have to wonder why Amazon offers "Stickerless Commingling" and "Stickered Commingling". What does the sticker add if the bin system is working so well?
That may be why they stopped calling the option commingled. They still use the word, but the option is just called "manufacturer barcode". It used to be called stickerless commingled inventory.
There's no stickered commingled option. There's stickerless commingled and stickered separate inventory.
One reason amazon keeps the stickered option is because some people might want to be able to get the exact inventory they send in back. And some items don't make sense to be commingled, like used items, items with experation dates, etc.