It looks like maybe it's not copying the sysupgrade firmware to either of the boot partitions?
root@OpenWrt:~# fw_printsys
bootpartition=1
SN=0987654321
dualfname1=openwrt-realtek-rtl838x-netgear_gs110tpp-v1-initramfs-kernel-v9.9.9.9.bin
root@OpenWrt:~# fw_setsys bootpartition 0
root@OpenWrt:~# reboot
Then, after a warm reboot, this output:
root@OpenWrt:~# fw_printsys
SN=0987654321
dualfname1=openwrt-realtek-rtl838x-netgear_gs110tpp-v1-initramfs-kernel-v9.9.9.9.bin
bootpartition=0
(Verified same with a cold reboot just in case, no change.)
FWIW: I just came across this very similar thread for a GS308Tv3. It seems like the fw_setsys bootpartition 0 worked for them.
EDIT 1:
I think what I might need to do now is re-attempt sysupgrade -n now that the system is set to partition 0 for boot. Certainly can't hurt to try.
EDIT 2:
I tried another round of sysupgrade -n and it made no difference. Same behavior, same fw_printsys output.