I checked and you are exactly right!
=> nmbm nmbm0 state
Physical blocks:
Legends:
- Good data block
+ Good management block
B Bad block
I Main info table
i Backup info table
M Remapped spare block
S Signature block
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------B-----
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
Logical blocks:
Legends:
- Good block
+ Initially remapped block
M Remapped block
B Bad/Unmapped block
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------M-----
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
Also, I was too hasty in my assessment that everything is working fine now.
After rebooting to check nmbm nmbm0 state, the router no longer boots successfully.
The symptoms are exactly the same as previous -- the device boot loops. But now with the serial console I can see why:
[ 3.748764] mt7530 mdio-bus:1f lan1 (uninitialized): PHY [mt7530-0:03] driver [MediaTek MT7530 PHY] (irq=26)
[ 3.761208] DSA: tree 0 setup
[ 3.767847] UBI: auto-attach mtd6
[ 3.771273] ubi0: attaching mtd6
[ 5.428779] ubi0 error: ubi_io_is_bad: error -77 while checking if PEB 516 is bad
[ 5.436552] ubi0 error: ubi_attach_mtd_dev: failed to attach mtd6, error -77
[ 5.443640] UBI error: cannot attach mtd6
[ 5.449232] /dev/root: Can't open blockdev
[ 5.453378] VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6
[ 5.460865] Please append a correct "root=" boot option; here are the available partitions:
[ 5.469189] 1f00 512 mtdblock0
[ 5.469201] (driver?)
[ 5.475730] 1f01 384 mtdblock1
[ 5.475741] (driver?)
[ 5.482259] 1f02 1024 mtdblock2
[ 5.482270] (driver?)
[ 5.488768] 1f03 1024 mtdblock3
[ 5.488777] (driver?)
[ 5.495296] 1f04 1024 mtdblock4
[ 5.495308] (driver?)
[ 5.501824] 1f05 4096 mtdblock5
[ 5.501835] (driver?)
[ 5.508335] 1f06 114816 mtdblock6
[ 5.508343] (driver?)
[ 5.514869] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[ 5.523110] Rebooting in 1 seconds..