Hi Zane,
Which authentication method are you using for the external data list? Web application pool is not available for end users in SharePoint Online. Based on the situation, I suggest you set the BDC object permission to All Users to see if there are any improvements.
To set the object permission, please see the steps below:
1. In the Administrator center of SharePoint Online, click Business Data Connectivity.
2. Select your BDC object, and click Set Object Permissions. Add All Users and grant all the permissions to the All Users group.
3. Set the Metadata Store Permissions of the BDC object, and grant full permissions to All Users.
After that, try to run your workflow again.
Thanks,
Reken Liu