[jpieper]

Yep, there is probably one clock domain for all the ADCs, although there are two different prescalers (one for ADC1/2 and another for 3/4/5).

I could see one of the writes getting lost. In this case though, the ADC enable is what seems to be timing sensitive, however the ADCs always end up enabled properly. It is just that a write that was significantly earlier (the one that sets the prescaler) seems to be lost, despite the register reading back that it was read correctly.

I would expect that if the synchronization failed, reads back would read the wrong value?