If you stick your objects on a grid, the grid takes into account the labels too. This is most clearly observed with actors in use case diagrams. As soon as you add a few letters to a label, it wants to have a different spot on the grid. Which is a bit odd as the key item is the actor symbol and not its label.
Contradictory however, the alignment tool works ok, by ignoring the length of the label. The alignment is based on symbol alone. Which makes a better diagram.

Dear Koos,

Thanks for your report.
We see the problem that you pointed out here. We will check into this issue and improve it in the future.

Thanks.

Satomi