This can be worked around by running
fw_setenv bootcmd "rtk network on; boota"
Reason: There is no LED support on RTL83xx in OpenWrt. This commands forces U-Boot to properly set up the LEDs.
NB: In older OpenWrt builds, fw_setenv doesn't work because the partition is not writable. In this case, you need to upgrade first, this was fixed some time ago.