It regenerates based on measured/calculated soot mass, oil ash volume, as well as the pressure measured in the DPF itself. Newer cars may have more stringent DPF regen detection/cycles vs their older counterparts, because if it doesn't catch it soon enough and a sufficient amount of time isn't provided for it to fully regen, then it can go as far as not regenerating due to the soot mass being too high, i.e. beyond a certain value.
When I have topped up the oil on the cars of customers I have in my part time job that isn't detailing related, I have personally found that newer cars are quicker to let you know the oil is low vs older cars. I've had no end of older cars come in where the oil light had only just come on, only to find it wasn't even measuring on the dipstick after I had topped it up with a 1 litre bottle. The same principle may be happening here with your DPF, but like yourself I'd like to hear more from other biTDI owners on their DPF regens.
