A stuck at fault in the control logic or in the IO interface.
A single "stuck at" fault that would disable a whole chip that is completely 4-way redundand?
You are incompetent.
The above is the proof.
There is probably some logic selecting which core(s) to use and some common IO interface. A single fault here could render the device useless depending upon the design. Give me the netlist and I'll show you where the possible faults are.