This is intended behavior as updating the coordinates for every single mouse move event would be too slow (the coordinates would be lagging behind when the mouse is moved around a lot).

How it that? I can't find any other CAD och graphics program that stops updating coordinates exactly when it is needed (when drawing)! Even Freecad does it right.

You must be misunderstanding me completely? If I move the mouse it does not update any coordinates at all. For minutes, if I keep going at it. This is very odd behavior. The ruler markers tracks the mouse perfectly, but the numbers are not updated.
The numbers updates (kind of quirky) on very small movements, but when the movements gets bigger it stops completely.

As far as I can remember QCAD updated continuously since I first bought it 15 years ago!
If it could be done 15 years ago, sure the computer should be powerful to do it in 2018!
I just tried 3.6.4.0 (3.6.4) from 2014 and sure the coordinates in the lower left corner updates just fine while drawing.

It is not like it has to be done every millisecond, a timer set at 100 or 200ms intervals would be just fine (the usual trick to save power). These simple calculations and updating some tiny text can not possibly be a particularly processor intensive unless you are doing something very strange.

Please bring this functionality back! It is pretty much useless for me without it.

I can confirm that coordinates should be updated at intervals even if the mouse keeps moving.

This was possibly caused by a Qt update (QTimer::start with interval 0 does not fire an update event anymore).