Thank you both for your help. It's solved with Daniel's patch. The missing flash space is back. Phew
Here is the relevant part of the Bleeding Edge (r49395) boot log if you're still interested:
root@nucom:~# dmesg | grep -i -e linux -e jffs -e partition -e nvram -e cfe -e mtd
-e rootfs -e kernel
[ 0.000000] Linux version 4.1.23 (user@host) (gcc version 5.3.0 (OpenWrt GCC 5.3.0 r49395) ) #3 Fri May 17 15:36:53 UTC 2019
[ 0.000000] board_bcm963xx: CFE version: 1.0.37-106.24
[ 0.000000] Kernel command line: root=/dev/mtdblock2 rootfstype=squashfs,jffs2 noinitrd console=ttyS0,115200
[ 0.000000] Memory: 59328K/65536K available (3118K kernel code, 137K rwdata, 768K rodata, 1280K init, 197K bss, 6208K reserved, 0K cma-reserved)
[ 0.543727] jffs2: version 2.2 (NAND) (SUMMARY) (LZMA) (RTIME) (CMODE_PRIORITY) (c) 2001-2006 Red Hat, Inc.
[ 0.619419] bcm63xxpart: CFE boot tag found with version 6 and board type 96328ang
[ 0.627309] bcm63xxpart: Partition 0 is CFE offset 0 and length 10000
[ 0.633946] bcm63xxpart: Partition 1 is kernel offset 10100 and length 14a678
[ 0.641273] bcm63xxpart: Partition 2 is rootfs offset 15a778 and length e95888
[ 0.648699] bcm63xxpart: Partition 3 is nvram offset ff0000 and length 10000
[ 0.655954] bcm63xxpart: Partition 4 is linux offset 10000 and length fe0000
[ 0.663210] 5 bcm63xxpart partitions found on MTD device spi1.0
[ 0.669300] Creating 5 MTD partitions on "spi1.0":
[ 0.674246] 0x000000000000-0x000000010000 : "CFE"
[ 0.680960] 0x000000010100-0x00000015a778 : "kernel"
[ 0.687851] 0x00000015a778-0x000000ff0000 : "rootfs"
[ 0.694743] mtd: device 2 (rootfs) set to be root filesystem
[ 0.704318] 1 squashfs-split partitions found on MTD device rootfs
[ 0.710718] 0x000000910000-0x000000ff0000 : "rootfs_data"
[ 0.718081] 0x000000ff0000-0x000001000000 : "nvram"
[ 0.724980] 0x000000010000-0x000000ff0000 : "linux"
[ 0.912322] Freeing unused kernel memory: 1280K (80400000 - 80540000)
[ 11.922782] jffs2: notice: (320) jffs2_build_xattr_subsystem: complete building xattr subsystem, 0 of xdatum (0 unchecked, 0 orphan) and 0 of xref (0 dead, 0 orphan) found.
[ 11.939827] block: attempting to load /tmp/jffs_cfg/upper/etc/config/fstab
[ 12.028666] jffs2: notice: (315) jffs2_build_xattr_subsystem: complete building xattr subsystem, 0 of xdatum (0 unchecked, 0 orphan) and 0 of xref (0 dead, 0 orphan) found.
[ 12.369683] block: attempting to load /tmp/jffs_cfg/upper/etc/config/fstab
[ 12.389603] mount_root: switching to jffs2 overlay
[ 19.002653] Loading modules backported from Linux version v4.4-rc5-1913-gc8fdf68
[ 19.329344] xt_time: kernel timezone is -0000
And with 18.06-SNAPSHOT, r7845-564d81e944 including Daniel's patch:
root@OpenWrt:~# dmesg | grep -i -e linux -e jffs -e partition -e nvram -e cfe -e mtd
-e rootfs -e kernel
[ 0.000000] Linux version 4.9.189 (user@host) (gcc version 7.3.0 (OpenWrt GCC 7.3.0 r7845-564d81e944) ) #0 Sat Aug 17 15:23:17 2019
[ 0.000000] board_bcm963xx: CFE version: 1.0.37-106.24
[ 0.000000] Kernel command line: rootfstype=squashfs,jffs2 noinitrd console=ttyS0,115200
[ 0.000000] Memory: 58920K/65536K available (3361K kernel code, 179K rwdata, 856K rodata, 1292K init, 208K bss, 6616K reserved, 0K cma-reserved)
[ 0.569329] jffs2: version 2.2 (NAND) (SUMMARY) (LZMA) (RTIME) (CMODE_PRIORITY) (c) 2001-2006 Red Hat, Inc.
[ 0.682298] 3 fixed-partitions partitions found on MTD device spi1.0
[ 0.688878] Creating 3 MTD partitions on "spi1.0":
[ 0.693806] 0x000000000000-0x000000010000 : "cfe"
[ 0.702126] 0x000000010000-0x000000ff0000 : "linux"
[ 0.712589] parser_imagetag: rootfs: CFE image tag found at 0x0 with version 6, board type 96328ang
[ 0.721958] parser_imagetag: Partition 0 is kernel offset 100 and length 16405c
[ 0.729473] parser_imagetag: Partition 1 is rootfs offset 16415c and length e7bea4
[ 0.737258] parser_imagetag: Spare partition is offset 900004 and length 6dfffc
[ 0.744878] 2 bcm963xx-imagetag partitions found on MTD device linux
[ 0.751428] Creating 2 MTD partitions on "linux":
[ 0.756255] 0x000000000100-0x00000016415c : "kernel"
[ 0.764953] 0x00000016415c-0x000000fe0000 : "rootfs"
[ 0.774219] mtd: device 3 (rootfs) set to be root filesystem
[ 0.781175] 1 squashfs-split partitions found on MTD device rootfs
[ 0.787586] 0x000000900000-0x000000fe0000 : "rootfs_data"
[ 0.796654] 0x000000ff0000-0x000001000000 : "nvram"
[ 0.990434] Freeing unused kernel memory: 1292K
[ 0.995109] This architecture does not have kernel memory protection.
[ 5.140996] kmodloader: loading kernel modules from /etc/modules-boot.d/*
[ 13.345162] kmodloader: loading kernel modules from //etc/modules-boot.d/*
[ 14.259506] jffs2: notice: (398) jffs2_build_xattr_subsystem: complete building xattr subsystem, 0 of xdatum (0 unchecked, 0 orphan) and 0 of xref (0 dead, 0 orphan) found.
[ 14.276756] block: attempting to load /tmp/jffs_cfg/upper/etc/config/fstab
[ 14.338199] jffs2: notice: (394) jffs2_build_xattr_subsystem: complete building xattr subsystem, 0 of xdatum (0 unchecked, 0 orphan) and 0 of xref (0 dead, 0 orphan) found.
[ 14.647443] block: attempting to load /tmp/jffs_cfg/upper/etc/config/fstab
[ 14.670692] mount_root: switching to jffs2 overlay
[ 17.361591] kmodloader: loading kernel modules from /etc/modules.d/*
[ 17.881488] Loading modules backported from Linux version wt-2017-11-01-0-gfe248fc2c180
[ 18.144591] xt_time: kernel timezone is -0000
Thanks again for your rapid help.