sefralgon:

The problem occurs on OpenWrt 19.07.0rc1 official image for this device, which is named
openwrt-19.07.0-rc1-ramips-rt305x-dir-300-b1-squashfs-sysupgrade.bin

If it's flashed on the target device, D-Link DIR-300 B1, using emergency tftp interface via UART, the result is following:

/dev/root /rom squashfs ro,relatime 0 0
proc /proc proc rw,nosuid,nodev,noexec,noatime 0 0
sysfs /sys sysfs rw,nosuid,nodev,noexec,noatime 0 0
tmpfs /tmp tmpfs rw,nosuid,nodev,noatime 0 0
tmpfs /tmp/root tmpfs rw,noatime,mode=755 0 0
overlayfs:/tmp/root / overlay rw,noatime,lowerdir=/,upperdir=/tmp/root/upper,workdir=/tmp/root/work 0 0
tmpfs /dev tmpfs rw,nosuid,relatime,size=512k,mode=755 0 0
devpts /dev/pts devpts rw,nosuid,noexec,relatime,mode=600,ptmxmode=000 0 0
debugfs /sys/kernel/debug debugfs rw,noatime 0 0

rootfs_data partition is not mounted.

dev: size erasesize name
mtd0: 00030000 00010000 "u-boot"
mtd1: 00010000 00010000 "devdata"
mtd2: 00010000 00010000 "devconf"
mtd3: 003b0000 00010000 "firmware"
mtd4: 00151973 00010000 "kernel"
mtd5: 0025e68d 00010000 "rootfs"
mtd6: 00040000 00010000 "rootfs_data"

Its size is only 4 erase blocks, 64k each. Apparently it's not enough for JFFS2 filesystem.