QCAD Professional
Win7 installer 32 bit
Version: 3.25.2.13 (3.25.2.13)
Internet: QCAD.org
Build Date: Feb 12 2021
Revision: cad2d36
Qt Version: 5.13.2
Architecture: i386
Compiler: Unknown

In the file provided one would expect *VARIES* in the property editor under custom properties when selecting 2 or more entities.
The key list is reduced to what keys are common to the multiple selection. (Here 3)
The values displayed are inconsistent depending the selection.

Select All >> 3 custom properties of line 3
Select the 3 vertical >> 3 custom properties of line 2
Select the first vertical and the diagonal >> 3 custom properties of line 1
Select all but the first vertical >> 3 custom properties of line 3
And so on ...

Regards,
CVH