Large nether portals are sometimes half-lit upon reloading a world

If i build a large portal (20 blocks by 20 blocks) (and so it crosses chunk boundaries), and also make a system which repeatedly breaks and replaces the portal, the portal can become half lit when reloading the world. (I was making a zombie pigman farm).

The portal can be broken by destroying the obsidian or by placing water in the lit area. The portal cannot be relit while the portal is only half lit. It still behaves like a regular portal.