Without seeing your project setup it’s hard to troubleshoot, however multi-reference fields either need to be included as their own collection list within a collection page, or nested within another collection list on a static page.
Another thing to look out for is scrolling within the source list as users on MacOS tend to have scrollbars hidden by default—meaning it may not be apparent that you can scroll through more options within the dropdown.
If both of those don’t seem to help, please include your read-only link and I’ll take a closer look 