I figured it out. In NuGet OpenXmlPowerTools has a dependency on OpenXMLSDK-MOT (>= 2.6.0) and pulls down v2.6.0 on install. What I did was to install the latest version of OpenXMLSDK-MOT which is v2.6.1. That caused the error to be thrown. But reverting back to v2.6.0 it worked.
I’m guessing there is a reference I could add to the config file to allow it to use 2.6.1, so I’m at the latest version.