“The exact same dd raw image that I’m writing to them should very well contain the same boot record and the same formatting, block alignments, and everything,”
I’m not sure that is true, especially the block alignments on different drives.
Rather than copying images to the drive, install a system to it. Get the ISO of the system you want and use something like RMPrepUSB or Easy2Boot to install it to the USB drive then see if it boots.
