Steps to reproduce:
In existing template with discovery add another item prototype as master item with JSON inside, after that convert existing item prototype from type "Zabbix agent" to "Dependent Item", select master item, than save.

Result:
Wait for discovery to fire and update existing items on hosts,
After that navigate to Configuration/Hosts and select items list from host with this template linked, here you can see errors(count depends on changed items count) at page end:
"Undefined offset: 0 [items.php:1582 ? expandItemNamesWithMasterItems() in include/items.inc.php:1773]"

affected items looks like:
"Disk discovery: : Disk sdb IO queue_ms"
note absence of master item between "::"
if you try to click on it: "No permissions to referred object or it does not exist!"
Try to disable\enable:
"Cannot disable item Details Incorrect value for field "master_itemid": Item "0" does not exist or you have no access to this item."

You can delete this item from host, after discovery It'll be correctly created, and will be functioning properly, but history data will be lost.

Expected:
Correct item conversion without data loss.