When two portals have been built parallel to each other (picture a), of any size other than the old standard, and then an Obsidian staff is built from top to bottom (picture b). When both portals have been lit, from background to foreground, the foreground portal renders strangely (picture c). (lighting of portal is independent, providing that the portal with the new stacked Obsidian is lit first.)

What I expected to happen was...:
portal to render correctly

What actually happened was...:
Foreground portal rendered but is influenced by a portal behind.

Re-creatable:
As per images:
Build a portal with dimensions other than the traditional 4 x 5

Build an Obsidian tower in one of the portals and light, then light the one in front. (this can be done with three parallel portals, etc.)