Moving/Copying activities between Cases

To replicate the issue:
On a clean install, create three cases using the default Housing Support case type (the case type isn't important).
In the first case created, choose one of the activities (I used the 'Follow Up' activity in my tests on the demo site), and select 'Move to Case' or 'Copy to Case'. In the resulting dialog box, a case will be pre-selected, and you won't be able to change it, where you should be able to choose any available case.
In my tests on the demo site - http://dmaster.demo.civicrm.org - the same was true when I tried to copy/move an activity to another case from the most recently created case, but if I try to move or copy an activity from the second of the three cases I was able to select the target case.
On our live site (running 4.7.15) we have numerous cases, five of which are ongoing, with the remainder closed. Three of the five ongoing cases are allowing me to move or copy activities to the case of my choice, whilst the other two cases will provide a pre-selected case in the move/copy dialog, and not allow me to select any other case as the target. All five cases have the same status.