Right. Sorry for the delay. I managed to track down what I think is causing it and video it.
https://youtu.be/KYdm6UVONN0
ChestSort has an option where you can shift+click to allow rapid entry of items into or out of a chest. In this scenario, one holds down shift and then rapidly double left-clicks. This seems to cause an issue with the Shulker Box, where it doesn't then register any entries into or out of it until it's next closed and re-opened. This can cause a loss of items put into it or duplication (if you take items out of it).
reported by anselhelm
Right. Sorry for the delay. I managed to track down what I think is causing it and video it.
https://youtu.be/KYdm6UVONN0
ChestSort has an option where you can shift+click to allow rapid entry of items into or out of a chest. In this scenario, one holds down shift and then rapidly double left-clicks. This seems to cause an issue with the Shulker Box, where it doesn't then register any entries into or out of it until it's next closed and re-opened. This can cause a loss of items put into it or duplication (if you take items out of it).
reported by anselhelm