You can ONLY reproduce if you have NVME drive (PCIe) that is mounted as data. My /boot directory is on the mmc, but in JP 4.6/4.6.1, bootloader tries to read /boot from NVME. Since my NVME is just data disk and doesn’t have /boot, bootloader fails to find /boot and decides to boot from kernel/dtb partitions. It never bother to try mmcblk0. In JP 4.3, bootloader reads mmcblk0 first and can find /boot.
I see this other user reported the same issue.