Either a question that has been asked too often, or not one that anyone recognises or cares to comment on.
Either way, for others in the same boat.
I'm wondering if it has something to do with the undervoltage warnings i'm seeing quiet a lot off. It just got me wondering having reread the boot issues sticky. I'm booting from one of three differrent options with varying cables(Phone power bank, 2600 LiPo and mains via apple plug (3 different cables but all generic), but it seems to be a common, but inconsistent issue. Although the pi always boots when attached to a screen - although the senshat stays 'rainbow' .
I dont know what the additional power draw a sensehat might induce, if any at boot time but its not my scripts because it doesnt get that far.