I have the following setup:
Are App service plan 1 and App service plan 2 in the same region?
You can have resources in multiple regions in the same resource group, but App Services and App Service plan must be in the same region.
My best guess is:
App service plan 1, App Service 1, and App Service 3 are in Region X.
App service plan 2 and App Service 2 are in Region Y.
You might have to recreate App Service 1a in App Service Plan 2. :(