Whether grid lock is on or not, the line tool would draw to the down and right of the intended position on the canvas.
In the demo, we first draw with the brush tool, then, the line tool.
We zoom in after and demonstrate more of the behavior with a circular brush, ascending in size.
We’ll observe that the anomaly happens every 1 interval of upsize.
PMNG v8.b8
Thanks for the report. Fixed with V8 beta 9.
Issue back in v8.b9
I couldn’t make how or when the issue happens. Sometimes, it works as is. Sometimes, it only works when you move it one pixel to the right, left, up, or down.
At times, when you click it at the same exact spot where it jumped, it won’t jump anymore and work as intended. Click somewhere else, do it again and click at that exact same spot, and it won’t work again. Real tricky.
The fix was not accurate. I had some mix up between x/y when rounding. The x-sub pixel coordinate was used for rounding the y coordinate 
Fixed with V8 beta 10
Thanks for the quick update, Jan! 