Geeetech

Made further tests.
Bed manually leveled with feeler gauges to 0.15mm.
After homing and running G29 (linear mode reading 4 points), Z0 now gives 0.25, 0.15, 0.1, 0.1 at the probe measured spots on the bed.
I think I will give this a rest until the firmware has been sorted. Reducing my active work ...

Thanks for the links - very interesting reading.
I am surprised that the bi-linear algorithm is not extrapolating to the un-probed areas. I think that maybe the linear option will be better.
Will run some tests today.

I have the same problem.
After performing a G28, G1 Z0, I carefully leveled the bed manually with feeler gauges to 0.2mm gap at each corner, and then rechecked after a further G28, G1 Z0.
I then performed a G28, G29, G1 Z0, and by issuing G1s to each bed corner I rechecked the gap and measured 0.2 ...