As stated above. The perimeterInfo command does not check blocks below y=0

Does the same apply above the old height limit? In other words,

1. Find some flat terrain below Y=128.
2. Fly above Y=256 in that area and place some blocks there. Example: /fill ~-10 300 ~-10 ~10 300 ~10 stone
3. Run /perimeterinfo while you are standing on the newly generated platform.

It also does not work for any blocks at y>=255.

Currently, these limits are hard-coded. If they are to be changed, they will need to be dynamically queried with a function instead of remaining hard-coded, as the build height limits are not the same in the Overworld, the Nether, and the End.