BQ28Z620: firmware update via bq.fs: initial ROM write NACK, device unrecoverable

Tool/software:

Bus/topology: Single-master I²C/SMBus (only our MCU; no EV2400/others)
Addressing: 7-bit. Before entering ROM mode 0x55; after entering ROM mode we send to 0x0B.
Intent: Program/upgrade FW from MCU using BQStudio-generated bq.fs / df.fs per TI FAQ

Sequence (from bq.fs):

; Unseal
W: AA 00 14 04
W: AA 00 72 36
W: AA 00 FF FF
W: AA 00 FF FF
X: 1000

; Go To ROM Mode
W: AA 00 00 0F
X: 1000

; First ROM-mode Data Block
W: 16 07 DE 83
X: 200
W: 16 05 12 00 00 19 00 0D 00 10 00 13 00 16 00 9B 97 70 9B 3B C1
X: 2
W: 16 05 12 10 00 9B 3E C1 9B 29 7C 9B A2 7C FC 60 30 18 0C 24 50

Problem: All commands ACK up to Go To ROM Mode. The next write W: 16 07 DE 83 is NACKed even with the file’s delays honored. After this point the gauge is not recoverable on our side.

Tried: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1288151/bq34z100-g1-how-to-exit-rom-mode

Request:

• Guidance on likely root causes for a NACK on ROM-mode writes.

• A step-by-step recovery procedure when the first ROM-mode write NACKs on a single-master bus.

• Any other suggestions to reliably program and upgrade the bq28z620 from an MCU.

Reference:   https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1107251/faq-bqstudio-how-do-i-interpret-the-lines-in-a-bq-fs-df-fs-file-for-use-with-my-i2c-or-hdq-gauge