I created following Custom Field : Group as Multi-Value Pick List

Then I started to add values in the Custom Field Group List. Then I selected multiple Movies and the Edit (all) function with the GROUP field selected for editing , and then “add” to add an first Group value to those movies.
Then I tried for the same movies to add a second Group value (still using the Add function) but the second value just replaced the first one (so the value was replaced instead of added).
However when I do the same by editing the details directly for each movie, I can add 2 values to the Group List.

