It sounds like you know what you are doing, and what you describe should work fine.
I haven't flashed a new Sparky2 in a while, but I did 3 and I even put one back to TL BL and retested several times. I suppose there could be a bug in the current next. I should probably put a Sparky2 back to TL BL and make sure the "new board procedure" still works like it used to.
The "new board procedure" you are doing doesn't need a good app firmware, just a good bootloader. You might have a bad bootloader (seller sent you a board that someone else flashed and returned). There may be a solder blob that needs removing somewhere. You may have a bad board.
Is your Linux running on bare metal or in a VM? I haven't tried it from a VM.
I would try a different computer and a different USB cable.