Looks like this was a problem with an earlier version of the app. The release notes for the current version (2.5.6 at the time of writing) indicate a fix for errors related to filtering on the "index" attribute for the data models and pivot interface. I believe if you update, it should be fixed!
Have the same issue, not only for "Interfaces" but also for panels: "Configuration started by host", "Device Logins by user".
Additionally, If it helps, I have installed an App "Syslog Priority Field Decoder / Lookup" for adding Severity level fields to Splunk and have configured "no_priority_stripping = true".