with some experience on sliding head machines you can take quite a meaty cut but it all depends on material, so sppeds and feeds to suit are crucial, normally with a heavy cut the feed would be a minimum F.03 MM/REV lets say, far quicker to take rapid 1mm cuts so its less stress on the machine and remember were machining holding on a guide bush which is only supporting the bar not clamping it like a chuck so material type is important on all aspects of speed feed and depth of cut, hope this helps
I have (4) 25mm Polygim swiss lathes and have never had the OAL vary from turning the OD. Check your main spindle collet pressure. The bar may be slipping in the collet causing the OAL variation. I usually run mine at 20psi. Are you cutting 12L14? I can turn 12L14 at a safe 450sfm and .002fpr without any issues at a DOC of .100" so like gollame said it might be the material.