Slurs will disappear in measures that have Slash Notation applied to the first part of the measure. This may be true of other Staff Styles as well, but I haven't tested it.
To make this bug appear:
Enter a half rest and two different quarter notes in a measure of 4/4. Add a slur to the two quarter notes. In the Staff Tool, drag across the first two beats of the measure and hit S to add the Slash Notation staff style. The slur disappears.