I had the exact same issue on my desktop at work. I tested my laptop and the issue was present, just as described above. Both machines running Oxygen 3a on Kde Neon.
However I did discover a workaround.
Changing GTK style options in KDE:
System Settings -> Application Style -> GNOME Application Style
I changed GTK2 and GTK3 theme to Breeze-Dark
I then applied the changes and restarted eclipse.
Now the menu and context menu were corrected.
I have replicated this on both my machines and another colleague’s workstation as well
Hope this helps