Seeking backwards breaks encrypted dash files

[Carl Eugen Hoyos]

The sample from ticket #6451 works fine, seeking backwards breaks playback. Depending on the seek, playback may or may not return.
Seeking back 10 seconds after 12 seconds playback (then quit):

$ ffplay cenc.mp4 -decryption_key 00112233445566778899AABBCCDDEEFF
ffplay version N-91217-g2bd24d4 Copyright (c) 2003-2018 the FFmpeg developers
  built with gcc 6.4.0 (GCC)
  configuration: --enable-gpl --enable-gnutls --enable-libx264
  libavutil      56. 18.102 / 56. 18.102
  libavcodec     58. 19.105 / 58. 19.105
  libavformat    58. 17.100 / 58. 17.100
  libavdevice    58.  4.100 / 58.  4.100
  libavfilter     7. 25.100 /  7. 25.100
  libswscale      5.  2.100 /  5.  2.100
  libswresample   3.  2.100 /  3.  2.100
  libpostproc    55.  2.100 / 55.  2.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'cenc.mp4':q=    0B f=0/0
  Metadata:
    major_brand     : dash
    minor_version   : 0
    compatible_brands: iso6mp41avc1
    creation_time   : 2017-04-13T19:17:09.000000Z
  Duration: 00:01:00.00, start: 0.000000, bitrate: 102 kb/s
    Stream #0:0(und): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yuv420p, 192x144 [SAR 1:1 DAR 4:3], 14 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
    Metadata:
      creation_time   : 2017-04-13T19:17:09.000000Z
      handler_name    : VideoHandler
      encoder         : AVC Coding
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7ff750000940] Incorrect number of samples in encryption info
    Last message repeated 1 times
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7ff750000940] Incorrect number of samples in encryption info
    Last message repeated 2 times
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7ff750000940] Incorrect number of samples in encryption info
    Last message repeated 2 times
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7ff750000940] Incorrect number of samples in encryption info
    Last message repeated 2 times

[Balling]

Okay, that is still happening. But it does not happen on sample from here: #9807

Strange.