Splitting a document with the newpage directive only renders the first page. The result is similar to how the Plant UML online server outputs a multipage image.

Default plantuml CLI behavior when rendering a multipage UML is the first page gets the name of the uml file with the appropriate extension, subsequent pages are numbered with a "_001.extension" pattern e.g. diagram.puml -> diagram.png, diagram_001.png, diagram_002.png

example multi-page PlantUML:

@startuml
Alice -> Bob : message 1
Alice -> Bob : message 2

newpage

Alice -> Bob : message 3
Alice -> Bob : message 4

newpage A title for the\nlast page

Alice -> Bob : message 5
Alice -> Bob : message 6
@enduml

Current workarounds look to be splitting into multiple UMLs in the text files, or including multiple UML files.