At first I was chasing why fw_setenv (see thread "fw_printenv works but fw_setenv does not after switching to linux-fslc-imx 5.10.69 from linux-fslc-imx 5.4.112") quit working after updating to linux-fslc-imx5.10.69 from 5.4.114, but further testing has shown it might not be related to fw_setenv itself as doing a flashcp to a MTD flexspi NOR device partition doesn't work either.
If I simply try to copy a file with flashcp I get the following:
flashcp -v ./imx8mm-evk.dtb /dev/mtd7
Erasing blocks: 11/11 (100%)
Writing data: 40k/40k (100%)
Verifying data: 10k/40k (24%)File does not seem to match flash data. First mismatch at 0x00000000-0x00002800
I've checked kernel .config and device tree settings between the 5.4 and 5.10 series kernel and can find no issues that would explain why this is happening.