Thanks for the report!

I did some digging and what you experience is probably* due to the unique way "current item" is handled in subject hierarchy because of the plugins. Let me first explain why this is. Because the subject hierarchy plugins do not have access to the widgets, the central, singleton plugin handler class is used to retrieve the "current item". This current item is the one that just has been right clicked, and the plugins need to 1. offer functions for that item in the context menu, and 2. perform the action on the item that was selected by the user. Worth noting that on multi-selection, currentItem (in the plugin handler) returns the first selected item, and currentItems return all. The combo box has a tree view inside, which, when the current item is asked for, gets the "current item" from this plugin handler.
https://github.com/Slicer/Slicer/blob/master/Modules/Loadable/SubjectHierarchy/Widgets/qMRMLSubjectHierarchyTreeView.cxx#L349
This is a remnant from SH1 (back in the SH1 times, there was only one SH tree view in the SH module, and there were no comboboxes and re-usable tree views), that definitely needs to be changed.

What I think would be best is if the tree view simply got the selected items and returned them. This would solve the problem you reported. The currentItem function would return the first selected item, and currentItems would return all of them.

(*) I did not build your MWE, and in this form will not. There are multiple, much easier ways to provide an example:

1. An external module which can be built separately and simply added to the additional module directories in Slicer
2. Even better, just a python code that I can copy-paste into the interactor

Thanks for the explanation. Will you work on the fix, or should I?

I believe I already implemented the fix.
I'll test it thoroughly and then commit. Unfortunately I became sick yesterday so not sure if I can get this done today, but soon!

I commited a fix
https://github.com/Slicer/Slicer/commit/468067bca94d6bf42b3b01254d3410229a2cf8d3
Please check out tomorrow's nightly and close the issue if it looks good.

Now it works as intended. Thanks Csaba!

Great! Can you please close this issue?