The layout of an horizontal association doesn't consider the width of the multiplicity to determine the minimum width required for drawing the association line. It neither reserves a minimum horizontal space on both sides for achieving an unambiguous layout about which side of the association is concerned.
For example, an horizontal aggregation:
Pond "xxxxxx" o- "o..*" Swan
results in the confusing diagram:
The 0..* multiplicity should appear unambiguously on the swan side, and the line shhould be long enough to allow xxxxxx and 0..* to be on the correct side, with some distance to the opposite side and without overlap.
The layout of the verical alternative o-- doesn't suffer of such a confusing problem.
For the time being, the expected layout can only be obtained with the workaround of using a blank label for manual padding purpose:
Pond "xxxxx" o- "o..*" Swan : " "