In my case it happened when i changed the injectors - from stock 320's to 630 dekas. Currently running on 4bar fpr, krkte 0,04693. Temin 0.3. Idle is smooth, no misfires. Only when car warms up requested lambda is 1.000, actual is slowly decrasing till it reaches 0.727, and stays there. Before car is up to temp stft are 0%, after it reaches operational temp they stays at -25%, no matter what. Maf stock 70mm with stock mlhfm, kfkhm only set to 1 because of open air intake. When i unplug the maf, stft's are operating in +/-7 %, and lambda holds its desired value. Tried changing tvub, but result is rough idle and misfires.

Pressure test the car, you have a leak.