Fear comes in all flavors and doesn't always make logical sense to pet parents. Fear can easily go unnoticed for many dogs because they show quiet signs of distress.
We cannot measure a dogs fear by the intensity of their behavior. One dog might quake like a leaf, with tail tucked, wide eyes, and activity try to hide. Another dog might stand still head lowered, tail low, ears slightly rolled down and back, and that's it. Both these dogs are displaying signs of fear or discomfort but to what level an external witness cannot say. Just because one dog isn't shaking, or fleeing, or fighting doesn't mean they are feeling any less fear or anxiety than the dog who does do these things.
Every dog guardian should take some time to get familiar with their dog's body language. Things like jumping and being hyper can often be mistaken for happy behavior when often it's actually a stress behavior referred to as fidget.
Pay close attention to your dogs ears, eyes, and brow. You will find so much more information about their emotional state there than by looking at their tail. Tail