Diagram items omitted during export -> layering issue?

[MSchreber]

Describe the bug

export to png omits some parts of the diagram

To Reproduce

1. create an activity diagram
2. Layering Activities on top of Object boxes to use as unconventional loops <- sorry of course not a clean way to build diagrams
3. export to png

Expected behavior

export diagram to png as shown in Apollon. <- so here with all objects in foreground object boxes in background <- see screenshot

Screenshots

left side: export, right side: view in browser immediately before export

Environment

 - OS: macOS 15.2
 - Browser Safari
 - Version 18.2