I think this analysis is spot on. How could it come to this? Remember how an incorrect UBOOT_OFFSET of 33792 just recently thrashed the u-boot on eMMC of @andypiper: Updating uboot? - #5 by josch
I fixed that by changing UBOOT_OFFSET to zero which is the correct offset for the eMMC. For the SD-card, 33792 is the correct offset but the imx8mplus is unable to load u-boot from the SD-card so this offset is rather theoretical. The problem is, that the script creating the system images does not respect the SD_BOOT=false for the imx8mplus and happily writes u-boot with an offset of zero to the image, overwriting the MBR in the process. This commit should fix that situation: mkimage.sh: prevent u-boot from being flashed for platforms that cannot load it from SD-card (075f8603) · Commits · Reform / reform-system-image · GitLab
Unfortunately, due to another issue with imx8mplus and kernel 6.10 (Mntre.com repository URL problem - #2 by minute) we are unable to build system images right now…