Andrew,
For more detailed description please refer to:
https://www.qcad.org/rsforum/viewtopic.php?f=33&p=40081#p40081

In the included screen grab below, left is when drawn cubic, closed periodic, from 4 points.
Right is when the file is re-loaded.

It is hard to select the incorrect shape when pointing near it ... Probably it is referring to the correct internal shape.
After drawing it explodes to a Null length polyline with 2 identical vertices.

I can't include the error in a file as it reverts on re-load.

The same issue exist for quadratic, closed periodic, from 3 points.

Regards,
CVH