  We study the gas kinematics traced by the 21-cm emission of a sample of six
HI$-$rich low surface brightness galaxies classified as ultra-diffuse galaxies
(UDGs). Using the 3D kinematic modelling code $\mathrm{^{3D}}$Barolo we derive
robust circular velocities, revealing a startling feature: HI$-$rich UDGs are
clear outliers from the baryonic Tully-Fisher relation, with circular
velocities much lower than galaxies with similar baryonic mass. Notably, the
baryon fraction of our UDG sample is consistent with the cosmological value:
these UDGs are compatible with having no "missing baryons" within their virial
radii. Moreover, the gravitational potential provided by the baryons is
sufficient to account for the amplitude of the rotation curve out to the
outermost measured point, contrary to other galaxies with similar circular
velocities. We speculate that any formation scenario for these objects will
require very inefficient feedback and a broad diversity in their inner dark
matter content.
