Hi, I talked to Lukas on IRC and we figured out the problem. Currently, the reform-tools package encodes a u-boot offset of 33792 bytes. But this is only correct when flashing u-boot on the SD-card from which the imx8mplus cannot even load u-boot. When flashing u-boot to emmc, the correct offset is simply zero bytes. Lukas also pointed out that in case of the imx8mplus, updating u-boot should only be done by people who really, really want to and know what they are doing. So I’ll be setting the EMMC_BOOT value for the imx8mplus in reform-tools from true to false. This will also stop reform-check from suggesting to update u-boot.
I’m really sorry for the mess this caused you. I’m sorry that this soft-bricked your pocket. 
I talked with Lukas about u-boot offsets originally in March this year and that’s also how the incorrect 33792 byte value made it into reform-tools: 2024-03-19.log