To reproduce:

1. create a world with the seed in the screenshot
2. go to the nether and teleport yourself to the coordinates on the screenshot
3. use a /fill command to remove all the lava
4.  notice the lava pool being cut off

Expected behavior would be that the lava pool is gently ended or that it replaces all netherrack untill the bedrock.

Also note the immense sive and height of the lava pool, and the proximity of three other such deep lava pools, which might be a bug on itself.