"Free space" partition is unformatable/unerasable/unmergeable from Disk Utiliy MacBookPro11,3 High Sierra AFPS

After having recently reduced the size of APPLESSD AFPS partition there is now a "Free space" partition between the previous and the next one. Unfortunately, it is impossible to resize, merge, erase or format the free space partition with Disk Utility. Actually merging is accessible through - button but it fails with an internal error saying a disk could not be found. The same in recovery mode and SIP disabled. Disk Utility doesn't find any error. 

"Free space" isn't displayed in Windows disk manager, instead the previous partition is displayed with its size before shrinking. Nonetheless it is displayed in Linux gparted, so maybe I could format it and it would then be again mergeable/erasable from Disk Utility but I am not sure of this.

Any suggestion about how to solve this issue ?

(now Boot Camp Assistant crashes after introduction panel, this may be related)

diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk4         70.0 GB    disk0s2
   3:                 Apple_APFS Container disk3         99.9 GB    disk0s3
   4:       Microsoft Basic Data                         75.5 GB    disk0s4
   5:       Microsoft Basic Data                         224.0 GB   disk0s5
   6:       Microsoft Basic Data NOTHING                 209.7 MB   disk0s6
   7:       Microsoft Basic Data UBUNTU                  208.7 MB   disk0s7
   8:                 Apple_APFS Container disk1         219.2 MB   disk0s8
   9:                 Apple_APFS Container disk2         96.5 MB    disk0s9

/dev/disk4 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +70.0 GB    disk4
                                 Physical Store disk0s2
   1:                APFS Volume APPLESSD                63.3 GB    disk4s1
   2:                APFS Volume Preboot                 19.4 MB    disk4s2
   3:                APFS Volume Recovery                517.8 MB   disk4s3
   4:                APFS Volume VM                      1.1 GB     disk4s4

sudo gpt -r show /dev/disk0
      start       size  index  contents
          0          1         PMBR
          1          1         Pri GPT header
          2         32         Pri GPT table
         34          6         
         40     409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
     409640  136660240      2  GPT part - 7C3457EF-0000-11AA-AA11-00306543ECAC
  137069880   58653384         
  195723264  195039232      3  GPT part - 7C3457EF-0000-11AA-AA11-00306543ECAC
  390762496  147456000      4  GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
  538218496  437450752      5  GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
  975669248     409600      6  GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
  976078848     407552      7  GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
  976486400       2048         
  976488448     428032      8  GPT part - 7C3457EF-0000-11AA-AA11-00306543ECAC
  976916480     188547      9  GPT part - 7C3457EF-0000-11AA-AA11-00306543ECAC
  977105027         32         Sec GPT table
  977105059          1         Sec GPT header
Welcome to the joys of a not quite fully functional Disk Utility in the land of APFS. To be fair, it's difficult in a GUI for the app to figure out exactly what you want to do.

I can't see the container list in the left pane of Disk Utility but I'm assuming that "APPLESSD" is living in an APFS container all of it's own. To reclaim that space you need to "grow" that container to gobble it up.

This page has a fair explanation of the process.

With APFS you are actually better to add a volume to the container rather than partition it. All the volumes in a single container share the space so you don't need to worry about getting the size wrong and adding and deleting volumes is a trivial task. Once your container is back to the right size select it in the left pane and then "Add APFS Volume" in the Edit menu.