In addition I downloaded the demo and community versions of QCAD to see if the same problem occurs - it does.
BTW, although the documentation seems to suggest that QCAD installs itself in /opt, my three versions reside in my home directory:-
Block door lh 620.dxf Outgoings 2008.ods
Digibox resetting.odt Pictures
Door 720.dxf qcad-18.104.22.168-1-demo.linux.x86
First Floor Copy 2.dxf regionset.c
First Floor.dxf Second Floor.dxf
Geneology Symtronics Wind-up page 2.pnm
House Ground Floor.dxf Whole House 2.dxf
Income 2008.ods Whole House.dxf
Makefile William Coomer Census 1891.pdf
Should I attempt to place them in /opt, or doesn't it matter?