I have decided to create this thread because I have noted various issues with Hopper Minecarts. I feel that these issues need to be addressed. The first bug I would like to note is that other players can open Hopper Minecarts on other islands, even when they are not trusted. This means a player can easily steal items from other islands, if hopper Minecarts are involved. The second bug relates to how Hopper Minecarts behave on the server differently than on single player. I have created an Imgur with two photos. This is the link: https://imgur.com/a/dwiQI. In the first image, I show that there is a stack if items in a shucker box. In the other image, a hopper minecart is shown under the shucker box. The hopper minecart is not being powered in any way. This means it should be taking items out of the shulker box, but it is not. In single player, this setup works completely fine, and the items are being taken out like normal. Though it does not work on this server. I have seen these bugs being brought up in chat, though I am unsure if these bugs are getting the attention they need. Therefore I am making this thread to raise awareness and hopefully come to a solution quickly.
Thank you for making this post, and I've already found a solution to the problem and its been fixed. EDIT: So, Minecart Hoppers are most likely not coming to be a feature after all, as allowing them also allows everyone to access any chest on any island. So it's the obvious choice of not allowing them. Sorry about that.