The sequence does not need the first reset, because it's not possible to follow the next step if you early reset (because reset stop dfu mode).
This is enough :
sudo /usr/local/bin/dfu-programmer at32uc3b1256 dump >dump.bin
sudo /usr/local/bin/dfu-programmer at32uc3b1256 erase
sudo /usr/local/bin/dfu-programmer at32uc3b1256 flash --suppress-bootloader-mem <.hex file path>
sudo /usr/local/bin/dfu-programmer at32uc3b1256 reset
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