There was still one wrong transform which wasn't spotted by the unit tests because the pattern group only had a translate transform on it. The unit tests have been modified to feature this.
Furthermore, after Input by @Moini (!323 (comment 585771832)), I changed the tangential offset to be limited to the pattern length (fine tune the position of the pattern). Since this doesn't make sense for stretching the pattern, the tangential offset does not change dx and the start position in this case anymore.