I am very excited to see that boolean operations on polylines have finally been implemented.
A very interesting addition. Well done

It is very related to the concept of enclosed areas .. Surfaces.
Started dozen attempts to code something similar myself and then addition was my main interest.
All attempts failed at some point.
Especially with self-intersecting, in ambiguous cases, with nodes on other shapes, co-existent nodes of different shapes and so on.
However,
In the attached file I try to subtract shape B from shape A.
Selected shape A .. OW .. ,F .. Indicated shape B and that is accepted .. An auxiliary shape is previewed over B.
If I then hover randomly over the drawing panel, essentially doing nothing, then QCAD will crash within the minute, typically 5-10 seconds.
It is not a large set of complex shapes and still the same happens when clearing the 'Preview' option.
QCAD 3.31.0 / Qt 5.13.2 / Windows7 32bit i386
Related Bug report: FS#2605
Regards,
CVH