I'm on the HVAC side of it and using ESTmep, not CADmep, but the only way I can reproduce that is to set the minimum stretch length on the pipe to something like 120" (see attached). It will not fill with anything less than what is set for the minimum stretch length. To get there to see if that's the case, right click on the relevant button, select button properties, select the itm and click the button properties button on the right, then select the options tab. If there is any value other than 0 in the Minimum Stretch Length, that will be the shortest pipe it will fill with.
If that isn't it, I'm not sure what it could be.