Hi Rob,
Thanks for being patience. While started to work on the reported scenario, we have noted that, the chart properties for multiple charts is not processed properly, so export throws problem. We have considered this as improvement and logged a feature request on this. Based on other logged tasks priority, this improvement will be available in any of our upcoming release. You can keep track of an improvement from the below feedback link.
If you have any more specification/precise replication procedure or a scenario to be tested, you can add it as a comment in the portal.
Regards,
Durga G