Multi-Instance Activity not working by setting string collection variable using rest API

**Describe the bug**
I complete a task instance with string collection variable (etc, userList=["A","B"]) before multi-instance activity，after querying task with process instance id and assignee by API ("/query/tasks"), we find that there's only one task, which assignee is blank, process definition multi-instance Activity settings is correct.

**Expected behavior**
there should be two task instance when i query by API ("/query/tasks"), assignee is A of one task instance ,the another one should be B.

**Code**
If applicable, add the code that you are using when having the issue.

**Additional context**
Flowable 6.5.0.1 release, database is mysql, standalone mode, not a embed one