As an Apple employee (not in any role related to the app store), and satisfied user of Happy Scale, I'm sorry to hear about the ranking loss.
On the other hand, it's in the nature of machine learning systems such as search that sometimes, a simple update of the underlying model can lead to drastic changes in search results (say, for argument's sake, that there has been a recent influx of apps using the word "tracker", which could lead to the word being considered essentially noise).
This can happen without any "bug" in the underlying system, even less a sinister agenda to undermine any developer or help another one. So while these possibilities can and should be investigated, ultimately search results will sometimes change.
On the other hand, it's in the nature of machine learning systems such as search that sometimes, a simple update of the underlying model can lead to drastic changes in search results (say, for argument's sake, that there has been a recent influx of apps using the word "tracker", which could lead to the word being considered essentially noise).
This can happen without any "bug" in the underlying system, even less a sinister agenda to undermine any developer or help another one. So while these possibilities can and should be investigated, ultimately search results will sometimes change.