Lane lenght jumps to the far end

I am using version 4.0.01 on linux (4.19.0-2-amd64) and Windows (win32 (6.1) x86_64).
I have created a BPMN Process design diagram having two lanes.
At a certain point, which I cannot reproduce, any attempt of resizing the length of the lanes randomly put the lane length to the far end of the canvas.
By resizing little by little I managed to put back the lane length close to the extent of the content. Nevertheless, if I save the diagram in a file, the image has the size of the (big) canvas instead of that of the lane. I have to resize it with a graphic editor (crop to the content).