This is awaiting a change in fullscreeninteractive/silverstripe-multiselectfield.
Here's the patch for MultiSelectField: https://github.com/fullscreeninteractive/silverstripe-multiselectfield/pull/1#issue-36749761
Merge branch 'hotfix/4.4.1' into develop
Merge branch 'Development' into develop
Merge branch 'release/5.0'