When I created a raid farm, the pistons destroyed the minecarts with hopper and dropped only the minecart and not the hopper with the items inside of the hopper. I have looked around the farm and still have not found them therefore concluding they are not dropped from being destroyed.
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Thank you for your report!
We're actually already tracking this issue in MCPE-55859, so I've resolved and linked this ticket as a duplicate.
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Probably it wasn't the pistons that destroyed the hopper minecart, but a trident pushed by the pistons.