Hi Tubby,
We had several sub idle events on early RB211s during cross bleed starts. If the TL of the supplying engine was not advanced far enough to ensure a health N3 then the HP bleed demand would drag the N3 below self sustaining rpm (about 60%). Once the rpm had decayed a bit further, then advancing the TL had no effect. The engine would continue to decay until flames could be observed from the tailpipe.