Calling group.replaceMembers() to replace a direct group with its members deletes instead of replaces

groovy:000> oldGroup.replaceMembers(newGroup.members.collect {it.subject})

2019-09-17 10:08:17,838: [main] INFO  Group.replaceMembers(1057) - existingMemberListSize: 14, deletedMemberCount: 1, groupName: unc:org:3103:middleware:admin, newMemberListSize: 13, addedMemberCount: 0, changedRecords: 1, operation: replaceMembers

newGroup has 13 direct person members. OldGroup has newGroup as its only direct member, so 1 direct member and 13 indirect. I wanted oldGroup to have identical membership to newGroup, so 13 direct members.

Calling oldGroup.replaceMembers() with the set of the 13 members did not work. It simply deleted newGroup as a member, without adding the people subjects.