Hi,
Confirmed.
Recently implemented Center of Rotation = Centroid of convex bounding polygon fails (3.31.2).
Also refer to this
conversation.
Probably because there is no size in Y and calculations (sum of products divided by factors) with zero make everything
NaN.
An edge case that is overlooked.
Similar when there is no size in X.
Similar when they were drawn at 45°.
The solution is to force the old default behavior.
See menu: Edit .. Application Preferences .. Edit .. Rotate
Select the method: = Center of bounding box.
Please file a Bug report on
QCAD Bugtracker 
The use of the Centroid was intended to ensures reversibility.
Fails the intention for these inline shapes drawn at an angle not [0-45-90-135-180-225-270-315-360] degrees.
But it is more complex then that, it can also fail to rotate at all.
Kinda expected failing but on another
level.
BTW:
With two Line segments of unequal length the center of rotation will never be in the middle of the void.
Unless you place the relative zero there and use the third option by preference.
Then it is more practical to use
RO with the center of rotation defined by Snap Middle Manual (
SN)
PS:
Although the approach with a Hull is a bit different.
This issue is also one of the reasons why the 2D Centroids methods that I contributed do not handle multiple shapes.
See menu Misc .. Information .. 2D Centroids.
Also refer to this
topic, summing wire centroids in this case.
Regards,
CVH