What do you mean by "position"? Is this a list view layout where each "position" is a line item record for a different item listed on that invoice or are you printing from a layout where each "position" is a row in a portal to line items?
Yes, a list (view) of Postitions, not a portal, because a invoice has more then one page.
It may be this known issue originally reported with FileMaker 11. I have no info confirming whether or not it exists in FMP 13:
For More Information see: lost line when text field is break across pages in particular condition
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
This is still a known issue in FileMaker Pro 13
What margins (Seitenränder) did you choose for print layouts?
I was dealing with same issues but with portals. Setting all margins to 1px might be a workaround. Fabrice Ricker is the guy who knows a great deal about printing. See more or ask questions in the FMM-Forum:
This issue has been fixed in FileMaker Pro 14.0.1.