Doing that would have caused problems since fdl2-dl.bin is not meant for this device. It can be used as FDL2 (a loader for flashing partitions), but you should still flash the real bootloader (lk.bin) to the uboot_a and uboot_b partitions.
Your phone probably detected that your uboot_a was not correct and used the correct bootloader from uboot_b, but if you had changed that to fdl2-dl.bin too, it wouldn’t have worked.
