Read this thread.
The summary of which would that the size of /dev/mtd0 grew over time from 0x20000 (128 KiB) to 0x40000 (256 KiB). And if you are accessing it from an older image the dtb (device tree binary, compiled from a dts - device tree source), is hard coded with the smaller size.
Obviously when you try to write a “u-boot-spl.bin.normal.out” that is larger than 128 KiB to a partition that is 128KiB, the SPL (Second Program Loader) firmware is corrupt because it is truncated at 128 KiB.