Yes, I also have an El Capitan machine, in addition to the Mojave one that I am currently on, and behaves the exact same way as you have just described. So, it seems to me that this is now the new default behavior.
As far as Terminal commands go, yes, there are ways to change the permissions on the invisible trash can under your user account.
If you want to experiment, you can try to do the following. It will delete your current trash can -- under your user account -- and force the system to create a new one when you reboot, and it will have the default ownership permissions. You can then experiment like before and determine if you get the same behavior. If so, then I would say again that that is just the way things are now.
As far as what permissions to use in order to get back the behavior that you desire, I am not sure, assuming that it is even possible at all.
1. In the Terminal, type the following:
sudo rm -ri ~/.Trash
2. Enter your Admin password when prompted.
3. When you see the prompt "examine files in directory /Users/armageddonx/.Trash?" . . .
Answer "yes". [without the quotes, obviously]
4. When you see the prompt "remove /Users/armageddonx/.Trash/.DS_Store?" . . .
Answer "yes". [without the quotes, obviously]
5. When you see the prompt "remove /Users/armageddonx/.Trash?" . . .
Answer "yes". [without the quotes, obviously]
6. If there are any other trash prompts regarding files or folders in the trash, answer according to what you want to do with them.
7. Log out of your Admin account, and then log back in again.
8. A new ".trash" folder should now be created with the default permissions.
If this info has been helpful, and/or has resolved your issue, please let me know. Thanks!