To reproduce

• create userParameter under Zabbix agent:

  UserParameter=test[*],echo $1
  

  Create test with item:
  
  with Char and key test["1,2"]
  Observe that '1,2' value is received correctly:
  
  Now try to do the same with zabbix_get:

  [vagrant@zabbix-lab ~]$ zabbix_get -k test["1,2"] -s localhost
  1
  

  2 - is the second argument now and ignored, despite there are doublequotes so all chars should be supported and ',' too.
  https://www.zabbix.com/documentation/3.4/manual/config/items/item/key