Nether Portal Anomaly

I managed to get minecraft working on another machine, and everything worked out fine. However, when experimenting with observers and nether portals, I noticed something strange: when a portal is activated, the observer doesn't the change. When the portal is broken, however, it's a different story. When the portal is broken, only then the observer detected it. I tried it a few more times and got the same result. I figure a possibility of the cause of this glitch is that it may be because I didn't have the observer facing the same block that I lit on fire.

Note: I am sorry that there aren't any pictures this time.