TP-Link WR902AC v1 also have this problem. Here's the output from OpenWrt 18.06, I've done a rollback because the router was unusable, and I have already installed some extra packages. I can install 19.07-rc0 again if it's needed.
BusyBox v1.28.4 () built-in shell (ash)
_______ ________ __
| |.-----.-----.-----.| | | |.----.| |_
| - || _ | -__| || | | || _|| _|
|_______|| __|_____|__|__||________||__| |____|
|__| W I R E L E S S F R E E D O M
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OpenWrt 18.06.5, r7897-9d401013fc
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root@OpenWrt:~# df -h
Filesystem Size Used Available Use% Mounted on
/dev/root 2.8M 2.8M 0 100% /rom
tmpfs 29.4M 576.0K 28.9M 2% /tmp
/dev/mtdblock4 3.1M 392.0K 2.7M 12% /overlay
overlayfs:/overlay 3.1M 392.0K 2.7M 12% /
tmpfs 512.0K 0 512.0K 0% /dev
root@OpenWrt:~# cat /proc/mtd
dev: size erasesize name
mtd0: 00020000 00010000 "u-boot"
mtd1: 00730000 00010000 "firmware"
mtd2: 00152620 00010000 "kernel"
mtd3: 005b0000 00010000 "rootfs"
mtd4: 00320000 00010000 "rootfs_data"
mtd5: 000a0000 00010000 "tplink"
mtd6: 00010000 00010000 "art"