Nether Portal Fails to Work Like a Nether Portal

I built this giant gold farm with giant nether portals. I finished it, lit the portals, then logged off for a while. When I logged back on, a large part of one portal had completely vanished, leaving only the left-most and right-most parts as nether portal blocks. What's more is that I tried breaking a block of obsidian to break the whole portal in an attempt to fix it, and after I broke the obsidian, the portal blocks were still there. I removed the entire bottom side of the portal other than underneath the portal blocks, and the portal blocks didn't break. They finally broke when I broke obsidian blocks directly below them.