Spike in APM New Network Access Connections

About every two weeks we do see a spike in “New Network Access Connections” which corresponds in an increased number of “Active Network Access Connections”. These connections seem to stay open indefinitely.

Is that normal behavior? I could not find any F5 article describing that behavior.

We are using running BIG-IP 12.1.5 Build 0.0.6 Final on an i4600