Sounds like a bug in the software to me. You might be able to define the nature of the bug more distinctly, since it may have to do with a mix-up between incremental and absolute positioning. You might play around with setting G90 mode in the controller before you enter or call out the length offsets, just in case it misinterprets the length offset as an incremental movement.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)