I just upgraded my AR300M to 21.02.2 and fw_printenv fails on me.
root@OpenWrt:~# cat /proc/mtd ; cat /etc/fw_env.config ; fw_printenv
dev: size erasesize name
mtd0: 00400000 00020000 "nand_kernel"
mtd1: 07c00000 00020000 "nand_ubi"
mtd2: 00040000 00010000 "u-boot"
mtd3: 00010000 00010000 "u-boot-env"
mtd4: 00fa0000 00010000 "firmware"
mtd5: 00210000 00010000 "kernel"
mtd6: 00d90000 00010000 "rootfs"
mtd7: 00a60000 00010000 "rootfs_data"
mtd8: 00010000 00010000 "art"
/dev/mtd1 0x0 0x10000 0x10000
Warning: Bad CRC, using default environment
It seems like /etc/fw_env.config gets set wrong somehow.
Changing it to mtd3 seems to fix things.