The propellers are new, Those have two weeks of use.
Is you "semi-desert terrain" having recognisable pattern?
In case your Spark is flying low, and the ground is to homogen for the VPS, or in case it is not bright enough, your bird may think, it is pefectly astill hovering in plave, but moving. Perhaps with some wind or tha own designed airflow!
In this video you can see what I mean,
Even if it is the reverse, a moving carpet, which the spark follows to stay "positioned" over the ground