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.
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