When you destory a planet and create anthor planet in the sector the nav hazard from the destoryed planet disapears

That's not a bug.

how is that not a bug

creating a planet will use 10% of any nav haz in sector.
so if someone has blown up 10 planets and thus created
100% nav haz...those stinkers....you can drive a ship in,
blow 10 planets, and you're clear for invasion...or whatever.

just part of the game.

You can also use this to avoid having navhaz in your base. Pop the max planets per sector, and then only blow up one planet at a time until you have the planets you want.