Flash command writes random junk data to SPI-NOR. Verification fails and one can spot blocks of junk data in read-out Flashrom via Buspirate5 binary mode flashes fine.
Sorry, there was no reason to blame. The issue was due to cable length and absense of shield. After I shielded the flashing TagConnect cable with copper tape, all went smoothly.
Thank you for the prompt responce and hacky & useful tool.