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:
@startuml
storage this_a_very_long_storage
@enduml
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.
Thanks again!