This isn't so much a real bug, but when effects that apply magic weapon (or similar) to a weapon fall off, they strip all prior effects of similar nature.
If I had magic weapon on a sword, and someone were to cast bless weapon, greater magic weapon, or something else along those lines you will see both fall off whenever the shorter duration is finished.
I'm no sure if there is a remedy for this or not, but if so it would be cool ;)
no remedy