Mopi accepts power on timer delay but does not boot pi

[tennisparty]

I am having an issue where the Mopi stops turning the pi on based on a power on time delay. The mopi superficially appears to be accepting the power on delays sent from the pi, I check these with a piece of python code and the delays are reading correctly. The boot schedule works for 3-4 days but on the 4th or 5th scheduled wake up time the Mopi is not booting the pi at dusk.

I have checked the syslogs at this time and there is nothing unusual.

I am still getting this error when writing the shutdown delay to the board:
[Errno 70] Communications protocol error on write word
I have tested this with and without the workaround suggested in previous issues here - [https://github.com/raspberrypi/firmware/issues/828#issuecomment-311262108]
Is this workaround still necessary or is there something else to consider? I cannot see any other fault in lots of testing.

Any thoughts would be welcome,
Thanks,
Sam