The template validation failed: 'The repetition action(s) 'Apply_to_each_2' referenced by 'inputs' in action 'Apply_to_each_PG_Approver' are not defined in the template.'.
This error appeared after I used rename on an action. It seems that rename does not cascade to other actions that were using the reference being renamed. One has to visit each of them and reapply the affected inputs.
In this case, I renamed "Apply_to_each_2" to be "Apply_to_each_PG_Approver", but Flow couldn't automatically go through each input with the "Apply_to_each_2" prefix and cascade the change.
This is a big pain! Lesson - Upon adding an action, immediately use rename or leave it at the default name forever.