Is there an internal algorithm for calculating UUIDs of a flash filesystem?
I have no idea how you managed to use tune2fs to change the UUID since it should only be able to manipulate ext2/3/4.

In any case, apparently exFAT have backup boot sector just like FAT(32). So my guess is you only managed to change the UUID in the primary boot sector but not the backup one. So when you unmount it the filesystem driver reverts the change according to the backup boot sector automatically.