A known erratum in the POSTAL3 bootloader (U-Boot 2017.09 variant) can send the eMMC into a continuous command retry loop. The chip never enters sleep mode. Result? in a 6mm x 8mm package—enough to hit 70°C.
Ensure your Postal 3 is set to the correct logic level (usually 1.8V for modern eMMC). Using 3.3V on a 1.8V rail is a common way to "fry" the chip. postal3 emmc hot