Not normal but maybe a simple problem. The slide lock mechanism is very simple. On the mag there is a little tab that is connected to the bb pusher that catches onto the slide lock bar. When there are several bb's in the mag the tab is slid downward and away from the slide lock. As bb's are consumed the tab slides upward until at the end of the bb's it contacts the slide lock bar, pushing it up and against the slide (locking it back).
It sounds like from your description that the slide lock bar is catching when there is 1 bb in the mag.
First try another mag, if the problem still occurs then check the slide lock bar mechanism. Take off the slide and load a mag with several bb's. The slide lock bar should not lift up. Try with no bb's, it should lift up. Try with 1 bb, if the slide lock bar still lifts up then it is catching too soon and you may be able to file the bottom tab (be careful) enough for this to not happen.
Be careful, personally I'd just live with the last bb problem it sounds like everything else works.
Reference:
Slide lock bar is part # 66 & 67 on this diagram -->
http://tech.airsoftcanada.com/KSC/ksc-g18cs.jpg
The little tab on the right hand side of yours is probably bent downward a little and catching sooner then later.