Correct, the downward angle is likely what is causing your grief. Eufy never really explained that well to buyers but it’s all about fine tuning the angle at which the cam is pointed. The lower it’s aimed the shorter the detection zone, but aim it too high and it’ll go off every time your neighbor across the street steps onto his porch.
Once you get a feel for it you may even be able to bring the cam height down a bit to suit your needs, but it’ll require some trial and error to find what works best for your particular circumstance.
If you find that you get the detection zone nailed but the camera view is not seeing the entire yard, adding another camera from a different vantage point might be the way to go. (I have a wireless doorbell on the left side angled looking to the right, with a eufycam on the right side that is looking towards the left. Between the two nothing gets by without me knowing.)