A well known reason for this behavior are bad blocks in device's BBL, which you happen to have:

$ mdadm --examine /dev/sd[bcdefg]1

/dev/sde1:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0xc
     Array UUID : f39f89d2:1bcd3d55:d173d206:d85b8bbc
           Name : homer2:1
  Creation Time : Sun Dec 2 12:04:24 2012
     Raid Level : raid5
   Raid Devices : 6

 Avail Dev Size : 5860528128 (2794.52 GiB 3000.59 GB)
     Array Size : 14651317760 (13972.59 GiB 15002.95 GB)
  Used Dev Size : 5860527104 (2794.52 GiB 3000.59 GB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
   Unused Space : before=1960 sectors, after=1024 sectors
          State : active
    Device UUID : a33aacb4:00d283ef:5715be52:a0678279

  Reshape pos'n : 491520 (480.00 MiB 503.32 MB)
  Delta Devices : 1 (5->6)

    Update Time : Tue Dec 5 18:30:50 2017
  Bad Block Log : 512 entries available at offset 72 sectors - bad

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

blocks present.

^^^^^^^^^^^^^^^^^

       Checksum : 978e30 - correct
         Events : 4830578

         Layout : left-symmetric
     Chunk Size : 512K

   Device Role : Active device 3
   Array State : AAAAAA ('A' == active, '.' == missing, 'R' == replacing)
/dev/sdf1:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0xc
     Array UUID : f39f89d2:1bcd3d55:d173d206:d85b8bbc
           Name : homer2:1
  Creation Time : Sun Dec 2 12:04:24 2012
     Raid Level : raid5
   Raid Devices : 6

 Avail Dev Size : 5860528128 (2794.52 GiB 3000.59 GB)
     Array Size : 14651317760 (13972.59 GiB 15002.95 GB)
  Used Dev Size : 5860527104 (2794.52 GiB 3000.59 GB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
   Unused Space : before=1960 sectors, after=1024 sectors
          State : active
    Device UUID : cabeb84a:b5df18c9:cb378062:be0f8998

  Reshape pos'n : 491520 (480.00 MiB 503.32 MB)
  Delta Devices : 1 (5->6)

    Update Time : Tue Dec 5 18:30:50 2017
  Bad Block Log : 512 entries available at offset 72 sectors - bad

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

blocks present.

^^^^^^^^^^^^^^^^^

       Checksum : 9f01ea9c - correct
         Events : 4830578

         Layout : left-symmetric
     Chunk Size : 512K

   Device Role : Active device 0
   Array State : AAAAAA ('A' == active, '.' == missing, 'R' == replacing)

This feature generally shouldn't be used, because its implementation is unfinished. Empty BBL's can be removed from every device by giving "--update=no-bbl" option to mdadm on assemble, but before that you must manually regenerate content for each block in BBL's and then manually zero the lists in superblocks.