After upgrading to Ubuntu 17.10, multiple instances of okular will no longer group under a single icon in the desktop dock.
I am running 17.10 under X.org, and if it matters: I am viewing PDF files with okular. All other applications I have tested work correctly. For example, multiple instances of gnome-terminal and emacs will group under a single item in the desktop dock. There was no problem with grouping for okular in 17.04.
Why is it important to have them grouped? It enables one to use a keyboard shortcut to switch between applications of the same type (Alt+|). It also makes it easier to switch between different applications (Alt+Tab) since there are fewer instances to cycle through.
How can I get okular instances to group correctly in 17.10?