I faintly remember an event somewhere in Australia I think.
Here to carrier for the G/S was working, but the modulation was U/S.
That will cause the G/S indicator to always stay dead center regardless of where the plane is in the vertical profile.
The autopilot/autoland then did a to steep descend, that would have caused a crash if the crew had not been alert with height versus distance.
To autoland is not really intended for following a signal that never changes. My guess it it tries to reduce power/pitch down until it "sees" a movement on the G/P. If the G/P stays in center, the plane will continue down.
No idea if this is the case in Irkutsk, but its one explanation.
Btw, in the australian incident the G/P U/S was in notam, and the G/P iden't was suppressed from the carrier... Anyone remenbers to listen for that ;-)