Thanks for your suggestion @Dalton_Ruer . I found out that the graph is not correct. As soon as I zoom in, or remove the Product, the graph looks normal. I downloaded the table data and tried it in a new app, but the spikes appear again.
Maybe it is because of the version. (February 2020) I had a similar graph earlier and after we upgraded i suddenly found those spikes in my stacked area graph. If someone wants to try it, the table data is attached.