hmm really? Using this script on an AnimatedSprite....
extends AnimatedSprite
func _ready():
print(self.frames.get_frame("default", 0).resource_path)
Pops out the following in the Output Console...
res://DaveDaDev/Tutorials/6 Curves/Sky.jpg
Is that not what you're looking for? It is the same as when I mouse over the frame in the Frame Editor. If it's not doing the same for you, can you paste what you're getting instead?