What is the relation between altitude and vertical speed?
The graphic is attributed to Flightradar24. Flightradar24 relies on transmitted data from the plane (via ADS-B usually) and it is not always perfect. See: Why do aircraft on Flight Radar 24 jump around randomly sometimes?

Back to your question, decreasing vertical speed (V/S) is not descending, it's just the rate of climb is becoming shallower, but still positive.

Factors that can cause the illustrated drop:


Bad data from Flightradar24.
Insufficient reading resolution (thanks @bogl).
Imperfect reading by the plane (see below).




V/S and altitude are measured by the static ports, if the plane's attitude is not ideal, say the plane is moving sideways slightly, each static port on each side will read differently.

The measured quantities are also different, one is a rate of change, and one is absolute reading, this may cause the V/S to lag behind the altimeter.