Thanks for all the advice everyone.
Since the issue arose, I've installed Archicad25.
This didn't solve it.
I've deleted the view from the layout and replaced it, but this didn't solve it.
The next point confuses me:
I dragged a copy of the section and placed it on the layout.
Not only does the copy show the windows correctly, but the original section shows the windows correctly too.
But:
I then deleted the copy of the section, but this caused the windows to disappear from the original section again.
If this is just a glitch, I'll redo the section and move on.
But if I'm doing something wrong I'd prefer to learn what I'm doing wrong.....
ArchiCAD24 - since Nov 2020
Revit - 2005 to 2020
Windows 10 Pro
2 screens: nvidia quadro RTX 4000