The following diagram, but using red, correctly renders with a red background in both PNG and SVG formats. Similarly, the same diagram using transparent correctly creates semi-transparent PNGs and SVGs without a background. But when the background color is white (or unspecified) it appears that PNGs get a white background as expected, but SVGs are left with no background set and are therefore transparent.
@startuml
skinparam backgroundColor white
rectangle "White Background"
@enduml