If you want to sync nFeed custom fields between JIRA instances and these fields are connected to the same database you should take into account that the new version of nFeed custom field(v5.13.0) does not accept an array of ids as a value.
Therefore, you need to convert ids to values. This article provides an example of the script which helps to sync the values of the nFeed custom field when each side uses different nFeed's versions.
Add this script to the Data Filter to send nFeed/element custom field's data
Receive nFeed custom field's data