Thanks for the report.
There is indeed an issue: the storage should be a rectangle with round border (and not a circle).
This is more obvious with the following example:
We are not tikz expert here, so it would be nice if you could manually patch the generated tikz code to change the rectangle to a round border rectangle, and post the result back here.
Does it sound possible to you?
With your working example, it would allow us to fix this in PlantUML code.