Since the APU Bleed Valve is motorised, it can only close because of a loss of DC control power to the Isolation Valve relay R7745 ("AIR SUPPLY APU VALVE CONTROL).
DC control power goes through the APU start latch relay R7730, the APU fire switch, the APU bleed valve overhead switch and the APUC (rpm>95%) circuit.
Have you tried a new APUC?
Are the rpms actually dropping to 95%? Does the APU know the engine is being started? i.e. Does the maintenance page show MEC (Main Engine Start) for OP MODE during start? Is it the same for more than one engine? (There are four separate inputs from the start circuits to the APUC).
Do you have any CMC snapshots?