Only I frames are correctly encoded during H264 video recording

Question

Only I frames are correctly encoded during H264 video recording

I am recording video using a raspberry Pi 2 and a camera module in a Python script using a package picamera. See the Minimum Example below:

import picamera
import time

with picamera.PiCamera(resolution=(730, 1296), framerate=49) as camera:
    camera.rotation=270
    camera.start_preview()
    time.sleep(0.5)
    camera.start_recording('test.h264')
    time.sleep(3)
    camera.stop_recording()
    camera.stop_preview()

results

The result is a badly encoded video:

first frame ok
in the next 59 frames, the scene is barely noticeable, almost all are green or purple (it is not clear what makes the change between the two colors)
frame number 61 is ok

Basically, only I-frames are correctly encoded. This was clarified by experimenting with various values of the intra_periodfunction parameter start_recording.