Not 100% sure, but I think this has to do with the OpenWRT factory.bin image setting the version to "2.0.0" for reasons described above in this thread. This becomes set in the devices "soft-version" partition when factory.bin is flashed. Sysupgrade can only modify the "firmware" partition (which overlaps the original "os-image" and "file-system" partitions). So after sysupgrade restores the OEM firmware, when you later try upgrade to the later OEM firmware, it doesn't work because the version is still set at "2.0.0" which is too high. Usually the OpenWRT firmwares use "0.0.0" so this doesn't happen.
It's a bit tricky to workaround. AFAIK one way to change it would be to change the "config" partition here to not read-only, rebuild+flash, then carefully hexedit the soft-version location in the /dev/mtd file. The other way would be read-change-write with an external flash programmer.
TFTP restore would also easily fix this, but lately TP-Link don't have that automatically enabled in some of their devices's u-boot firmware, and it requires serial console access