Steps to reproduce:

1. Add HTTP agent item on template A, which calls json api which returns the following data

[{"name":"DEVICE1"},{"name":"DEVICE2"}] 

1. Add JavaScript preprocessing step with following code on the item defined above (which adds the groupname attribute to each of the elements in the json array returned above, from the {$ZABBIX_HOSTGROUP} macro):

newValue = JSON.parse(value);
var groupName = '{$ZABBIX_HOSTGROUP}';
for (i = 0; i < newValue.length; i++)
{
 newValue[i].groupname = groupName
}
return JSON.stringify(newValue);

1. Add discovery rule on template A, with the following LLD macro (extracts the groupname we added above to the {$ZBXGRP}):
   
2. Add host prototype to the discovery rule, while adding the {$ZBXGRP} macro to the "Group Prototypes"
   

Result:
The host prototype does not get assigned to the group, the preprocessing step works if i look in the "Latest Data" tab, so the problem is probably with the LLD macro, or the group prototypes
Expected:
The host prototype should be assigned to the hostgroup, as defined in {$ZBXGRP}