Squashfs errors even though I'm using extroot from day 1

The router is a TP-Link Archer C6U v1

dmesg is getting spammed with these lines:

[457981.052612] SQUASHFS error: Unable to read page, block 4f7552, size 20df4
[457982.072831] SQUASHFS error: Unable to read data cache entry [4f7552]
[457982.079293] SQUASHFS error: Unable to read page, block 4f7552, size 20df4
[457982.086323] SQUASHFS error: Unable to read data cache entry [4f7552]
[457982.092775] SQUASHFS error: Unable to read page, block 4f7552, size 20df4
[457983.112252] SQUASHFS error: Unable to read data cache entry [4f7552]
[457983.118721] SQUASHFS error: Unable to read page, block 4f7552, size 20df4
[457983.125719] SQUASHFS error: Unable to read data cache entry [4f7552]
[457983.132157] SQUASHFS error: Unable to read page, block 4f7552, size 20df4
[457984.152282] SQUASHFS error: Unable to read data cache entry [4f7552]
[457984.158759] SQUASHFS error: Unable to read page, block 4f7552, size 20df4
[457984.166501] SQUASHFS error: Unable to read data cache entry [4f7552]
[457984.172958] SQUASHFS error: Unable to read page, block 4f7552, size 20df4
[457985.193181] SQUASHFS error: Unable to read data cache entry [4f7552]
[457985.199640] SQUASHFS error: Unable to read page, block 4f7552, size 20df4
[457985.206630] SQUASHFS error: Unable to read data cache entry [4f7552]
[457985.213068] SQUASHFS error: Unable to read page, block 4f7552, size 20df4
[457986.232234] SQUASHFS error: Unable to read data cache entry [4f7552]
[457986.238700] SQUASHFS error: Unable to read page, block 4f7552, size 20df4
[457986.245683] SQUASHFS error: Unable to read data cache entry [4f7552]

even though since I bought the router, I've installed openwrt and configured extroot to a usb thumbdrive. I can't think of a reason for those error messages. I've had similar issues with other routers I've flashed with a custom image made with imagebuilder, although not this severely. The flash overlay partition is actually mounted even though it's not being used, I've checked with lsof and no process has any handle on any file there.

What is common between all the routers I've flahsed with imagebuilder images are these lines in dmesg that appear as soon as the device boots:

[   20.743825] jffs2: notice: (2272) jffs2_build_xattr_subsystem: complete building xattr subsystem, 24 of xdatum (23 unchecked, 1 orphan) and 34 of xref (1 dead, 0 orphan) found.

With the numbers creeping up until extroot is set up.

More info:

/etc/config/fstab

config global
        option anon_swap '0'
        option anon_mount '0'
        option auto_swap '1'
        option auto_mount '1'
        option delay_root '5'
        option check_fs '0'

config mount 'overlay_boot'
        option device '/dev/mtdblock4'
        option target '/overlay-boot'
        option fstype 'jffs2'

config mount 'overlay_usb'
        option target '/overlay'
        option uuid '3390558d-9557-4046-884f-d3afe7d6109e'
        option enabled '1'

config swap 'swap_usb'
        option uuid 0b37b65c-7d42-4e29-ae21-bcb052c62332
        option enabled 1

Command used to build the image:

make image PROFILE="tplink_archer-c6u-v1" PACKAGES="luci-ssl -wpad-basic-wolfssl wpad-wolfssl zerotier block-mount kmod-fs-ext4 e2fsprogs fdisk kmod-usb-storage-extras"

OpenWRT version: OpenWrt 21.02.3, r16554-1d4dea6d4f

What could be causing these issues?

I've discarted the idea of this being an issue with my builds. The same thing happens with the firmwares stock openwrt images. I've found this ticket where this isssue is discussed:

And this post where the same issue is mentioned:

However I'm not sure how to proceed from here. Any help is much appreciated.