Hi @Chris Lupton,
The reason you can create a Logic App against an App Service Plan in a different resource group but can't move an existing Logic App there is due to Azure's design. When creating a new Logic App, Azure allows selection from any resource group in the same subscription, as long as the region and OS type match.
Moving an existing Logic App enforces stricter rules to maintain consistency and avoid issues with dependencies. To move your Logic App to a different App Service Plan in another resource group, you would need to recreate it in the desired location.
If you have any further questions, feel free to ask!