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LTC6810-2 Can't write to configuration or PWM registers

I have a test board I have setup to monitor up to 12 cells using two LTC6810-2 battery management ICs, and I have a weird problem with the bottom 6-cell IC. The schematic is a bit different as the end application needs to be able to switch between 3 cell, 4 cell and 12 cell setups automatically. To that end there are some P channel MOSFETs changing the input configuration on the lower IC, and the two LTC6810-2 ICs are on the same SPI bus with a comms isolator between them for the voltage translation to the upper IC (see attached schematic of the dev board). The comms isolator basically open circuits the comms to the second IC when no battery balance lead is connected for that IC.

This mostly seems to work well, I can read registers from both ICs by addressing them, broadcast or address to start AD conversions, get cell voltages etc - but for some reason I can't write the config or PWM register groups of only the lower IC (the one not isolated). I found that if I disconnect the upper 6 cells (7-12) from the board, it suddenly works, and reconnecting again stops it working.

Looking at the 0-scope capture of the SPI comms, there doesn't seem to be any difference between the write communication when the upper 6 cells are connected vs disconnected. So the question is, what other conditions might cause everything to be readable but not writable that I can look into?

Schematic and o-scope screen shots attached - first is the write comms for an unsuccessful write (quick wake-up comms prior is not shown):

And the resulting comms from read back:

Next is the successful write (as far as I can tell this is identical to the unsuccessful write:

And the successful read-back of what was written:

I have since tried writing the REFON bit as well but this hasn't made any difference, I think the problem must be hardware related - bit stumped as to what might cause this. Schematic for reference:


Photo of the board: