ASL, AGL and ASF are the same above 27km ASL

As there is an achievement "dive below 14km" is was trying to find the deepest point in the ocean without google. My idea was just use Vizzy. All i need is there. So my program simply wrote into flight log the following stats:

while(true){
1 Text "ASL-ASF # ASL # ASF"
2 Ocean depth (ASL-ASF) in whole meters
3 ASL in whole meters
4 ASF in whole meters
5 wait a second
}

The program itself works fine, but above 27km ASL all the three altitudes start being the same.

As you can see in screenshot middle. Last useful ocean depth is -669m - the difference from 27570m ASF to 26901m ASL. While climbing up, every data point after is 0.

AGL does the same.