How to force AFPS to free up space?

My AFPS volume won't free up space after I delete even very large files.

Does anyone know how we can force it (with CLI) to effectively delete files?

Here's what Sierra's "diskutil list" booting off of /dev/disk3 has to say about my now-useless APFS volume:

Jasons-MacBook-Pro:~ moo$ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS                         999.7 GB   disk0s2
   3:       Apple_KernelCoreDump                         655.4 MB   disk0s3
/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme                        +999.7 GB   disk1
/dev/disk1s1 (internal, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                APFS Volume MBP-1TB-SSD            +991.2 GB   disk1s1
/dev/disk1s2 (internal, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                APFS Volume Preboot                +18.2 MB    disk1s2
/dev/disk1s3 (internal, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                APFS Volume Recovery               +507.9 MB   disk1s3
/dev/disk1s4 (internal, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                APFS Volume VM                     +1.1 GB     disk1s4
/dev/disk2 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *63.9 GB    disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:          Apple_CoreStorage Untitled 1              63.0 GB    disk2s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk2s3
/dev/disk3 (internal, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS SD Card 64Gb           +62.6 GB    disk3
                                 Logical Volume on disk2s2
                                 7BB1C619-BDDD-433F-9D69-84B72524F577
                                 Unencrypted

Also, when I was still able to boot into High Sierra, Disk Utility said there was no problem with my SSD:

Verifying file system.
Volume could not be unmounted.
Using live mode.
Performing fsck_apfs -n -l -x /dev/rdisk1s1
Checking volume.
Checking the container superblock.
Checking the EFI jumpstart record.
Checking the space manager.
Checking the object map.
Checking the APFS volume superblock.
Checking the object map.
Checking the fsroot tree.
Checking the snapshot metadata tree.
Checking the extent ref tree.
Checking the snapshots.
The volume /dev/rdisk1s1 appears to be OK.
File system check exit code is 0.
Restoring the original state found as mounted.
Operation successful.

BUT, when booting into High Sierra's recovery mode, it refused to perform a repair:

Verifying storage system
Storage system check exit code is 8.
Repairing storage system
Storage system check exit code is 8.
Storage system verify or repair failed.
Operation failed…

Disk Utility in Sierra and High Sierra both fail to perform a repair with the "Storage system check exit code is 8" message.