The original photos from the announcement in April show headphone jacks.Here is another hypothesis: the FM3s may have been initially designed to not have a headphone jack and perhaps during some market research or alpha testing, Fractal discovered a headphone jack is more desirable than they originally anticipated. At some point between ordering these "first-run" or "prototype" units and the press release the design was changed to include the headphone jack.
https://www.guitarworld.com/gear/be...ew-fm3-compact-amp-modelermulti-effects-pedal