I’d guess the distro image you’re trying to use is not detected via the u-boot bootscripts so it falls-thru back to Android. You can exorcise vendor u-boot from eMMC with:
echo 0 > /sys/block/mmcblk1boot1/force_ro
echo 0 > /sys/block/mmcblk1boot0/force_ro
dd if=/dev/zero of=/dev/mmcblk1 bs=1m count=10
If the distro image has it’s own u-boot then it should now boot from SD (as we just erased the first 10MB of emmc). If you need vanilla u-boot you can use https://chewitt.libreelec.tv/testing/u-boot/u-boot.bin.sd.bin-radxa-zero which is 2021.07 with some patches to support the zero.
If the goal is to make something work with the vendor u-boot you’ll need to share UART output so people can see where things go wrong.