Heres my effort
Here goes my attempt at explaining this situation.
The whole rationale behind the disconnect is based around 'dual channel' redundancy operation. It would appear that the operation on single channel was not considered worth
isolating as we can all fly manually down to CAT1 limits.
Why do I think this?
Background:
Defs:
FCC - Flight Flight Control Computer
IFSAU - Intgrated Flight System Accessory Unit
It is all down to the DFCS Power Transfer Interface,this unit uses the following components:
FCC A and B
Bus power control unit
IFSAU
Standby power control unit.
These in turn control the Bus Tie Transfer.
During a dual channel operation, each FCC must be on an isolated power source. FCC A gets dc power from DC bus 1 and FCC B gets dc power
from DC bus 2. Normally, the two dc buses are connected with the bus tie relay. If either FCC is in the active G/S mode (ie when glides
slope captures),it will cause the bus tie relay to open. This will separate the DC power sources to the two FCCs.
What happens when the Bus Tie Relay Opens and why does it open?
When EITHER the 'A' FCC or the 'B'FCC is in the G/S active mode it tells the IFSAU (just a control and comparison computer)
to isolate the DC Buses. The IFSAU needs confirmation from the Standby Power Control Unit that the power is normal by providing a ground
to the IFSAU.This causes the G/S engaged position to be activated in the IFSAU and cause the ground to be removed from the bus tie relay.
If the Bus Tie Relay has a ground from the IFSAU then the relay closes and the DC Buses are connected together, when the ground is
removed the rely opens and the buses are isolated.
The Bus Power Control Unit which comprises of the TR's usually get their power from different 115v ac buses (AC Bus 1 & 2).If the TR's
suddenly get their power from one source ie when a transfer occurs the Bus Transfer Control Unit knows where the source of power is from
(either the generators or the APU generator).
It therefore knows that even though both buses are powered it is from a single source. It therefore sends a signal to both FCC's that
the DC Buses cannot be isolated. Both FCC's now removes the flight director as it does not have its required redundancy for the Dual Channel approach.
Summary
To answer your question, why does it happen?
The Bus Power Control Unit knows where the power source is from that is supplying both buses, it knows it is a single source from the APU
generator so it tells BOTH FCC's that the buses cannot be isolated. The important point here is that it tells both, so even on a single channel approach the FCC fails the
Flight Director.
And that is why I think it happens.