Using framerate_delta freezes camera module until reboot

[mikhailkin]

I want to change framerate on the fly. From time to time Raspberry 3B with camera V2.1 I get module freeze. Nothing helps, but reboot. Code to reproduce:

import picamera
import time

print('Initializing camera')
camera = picamera.PiCamera()
time.sleep(0.5)
camera.resolution = (640, 480)
camera.framerate = 10
camera.annotate_frame_num = True
print('Ok')

n_cycles = 10

camera.start_recording('test.h264')
for i in range(n_cycles):
    if i > 2 and i % 2 == 0:
        camera.framerate_delta = -9.5
    else:
        camera.framerate_delta = 0.
    msg = f"{i}: {camera.frame.index:4d} F{float(camera.framerate_delta):2.1f}"
    print(msg)
    camera.annotate_text = msg
    camera.wait_recording(1.)

camera.stop_recording()

Output:

Initializing camera
Ok
0:    0 F0.0
1:    4 F0.0
2:   14 F0.0
3:   24 F0.0
4:   34 F-9.5
5:   36 F0.0
6:   36 F-9.5
Traceback (most recent call last):
  File "framerate_delta_issue.py", line 21, in <module>
    camera.framerate_delta = 0.
  File "/usr/lib/python3/dist-packages/picamera/camera.py", line 2417, in _set_framerate_delta
    self._check_camera_open()
  File "/usr/lib/python3/dist-packages/picamera/camera.py", line 538, in _check_camera_open
    raise exc
  File "/usr/lib/python3/dist-packages/picamera/camera.py", line 1995, in _control_callback
    "No data recevied from sensor. Check all connections, "
picamera.exc.PiCameraRuntimeError: No data recevied from sensor. Check all connections, including the SUNNY chip on the camera board

I was not able to reproduce issue on Pi Zero with different camera module. But I've seen same issue a couple of times.
If someone knows safer method to change framerate, please, share it.