Maximum of metrics in Grafana dashboard?

I am editing my Dashboard in Grafana. My Datasource is a REST-API, called by the JSON Datasource Plugin. To do this, the API Server responds with a list of metrics like this: ["one", "two", "three", "four", "five", "six", "seven", "eight"] , but somehow only the last three metrics of the list are shown in the dropdown menu.

dropdown menu in dashboard editor mode

On the other hand if I switch to explore mode, every metric of the list is shown regularly in dropdown menu.

dropdown menu in explore mode

Anybody having an idea, what I am doing wrong?