Apparently the trash unix/linux-desktop specifications don’t really allow for trash to work across different partitions. It even tries to actively prevent it by checking whether the trash folder is a symlink or something.
Some implementations do follow that completely (trash-cli), while others don’t and can be “cheated” (KDE GUIs on latest KDEs/QT4/QT5, I think), while others will apparently just delete the file without moving it to trash (the CLI command kdetrash, funnily enough).
That I don’t understand. While It’s completely reasonable that it wouldn’t do that by default, as it would just make everything needlessly slower, I think ideally it should allow for “hacking” the system with symlinks. I think I may have read of someone having hacked this with a bind-mount, though, even though I don’t even understand how would that be from this basic description.