This is about the bootloader, and has nothing to do with the initramfs version you used to install OpenWrt. The bootloaders are basically the same, but the one dated Jul 06 2021 is hexedited to reserve 16MiB ram to extract the uImage to. Your own build initramfs doesn't boot because it extracts to >8MiB.
As far as I know this cannot have impact on your problem. The kernel (+optional initramfs) boots or not. When the kernel boots u-boot role is over.