I just tried both shapes under Ubuntu 18 and macOS 10.14 and they worked as expected.
Did you try with Sweet Home 3D version available at SourceForge.net?

That's encouraging, so I spent another hour trying everything possible to produce an L-shaped hole in the ceiling for my staircase - without success. I've tried VH encoding, L encoding, my required measurements, the example measurements and a simplified rectangular hole - all to no effect. Always, the ceiling hole is the full rectangular base dimensions. It is not possible to make it smaller. The SVG encoding seems fine. It looks like a bug at the moment. Maybe because the design file is too big? I'm at a loss to explain this lack of custom hole ability.

I'm using the latest version of SH3D, i.e. 6.5 and I've looked inside the SH3F zip file and the SVG line is preserved as expected.

Can you see if you can make an L-shaped hole in this SH3D file with this SH3F staircase?

sh3f file
https://drive.google.com/file/d/1jIwirV ... sp=sharing

sh3d file (this is crazy big, even though it is basically one floor of 5 rooms).
https://drive.google.com/file/d/1w8UFLf ... sp=sharing