ubifs+overlayfs:Filesystem usage keeps increasing


root@OpenWrt:/# mount
/dev/root on /rom type squashfs (ro,relatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,noatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,noatime)
cgroup2 on /sys/fs/cgroup type cgroup2 (rw,nosuid,nodev,noexec,relatime,nsdelegate)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime)
/dev/ubi0_4 on /overlay type ubifs (rw,noatime,assert=read-only,ubi=0,vol=4)
overlayfs:/overlay on / type overlay (rw,noatime,lowerdir=/,upperdir=/overlay/upper,workdir=/overlay/work)
tmpfs on /dev type tmpfs (rw,nosuid,noexec,noatime,size=512k,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,noatime,mode=600,ptmxmode=000)
debugfs on /sys/kernel/debug type debugfs (rw,noatime)
none on /sys/fs/bpf type bpf (rw,nosuid,nodev,noexec,noatime,mode=700)
pstore on /sys/fs/pstore type pstore (rw,noatime)
root@OpenWrt:/# df -h
Filesystem                Size      Used Available Use% Mounted on
/dev/root                 5.3M      5.3M         0 100% /rom
tmpfs                   496.3M     48.0K    496.3M   0% /tmp
/dev/ubi0_4              79.1M    200.0K     74.8M   0% /overlay
overlayfs:/overlay       79.1M    200.0K     74.8M   0% /
tmpfs                   512.0K         0    512.0K   0% /dev

When I repeat the following steps, the occupancy of /dev/ubi0_4 keeps increasing.

jffs2reset -y && sync && reboot

I added some logs to the ubifs driver and found that the ubifs_budg_info->old_idx_sz parameter keeps increasing.

Why is this happening?