Flowable expression.getValue

Hi experter,
It seems the variable can not be separated with a comma.
For example, assigneeUser1=mike,tom,mary. It can only get the first value (mike) using the below function.
However, if assigneeUser1=mike; tom; mary separated by the semi-colon. It is working well.

Expression expression = expressionManager.createExpression(expressionText);

return expression.getValue(variableScope);

I am wondering why?
Could any expert explain it to me?