If mouse entered only fires when the mouse enters, then i would assume it would only flip the value when you enter. Maybe you want to flip to true when entered and flipped to false when exited, then you might want to do something like..
var camera_flipped := false
func _on_mouse_entered() -> void:
camera_flipped = false
$"../TextureRect2".visible = camera_flipped
func _on_mouse_exited() -> void:
camera_flipped = false
$"../TextureRect2".visible = camera_flipped
Doing it this way will also protect you if mouse_entered fires multiple times