Ultra-diffuse galaxies (UDGs) in the Coma cluster have velocity dispersion profiles that are in full agreement with the predictions of modified Newtonian dynamics (MOND) in isolation. However, the external field effect (EFE) from the cluster seriously undermines this agreement. It has been suggested that this could be related to the fact that UDGs are out-of-equilibrium objects whose stars have been heated by the cluster tides or that they recently fell onto the cluster on radial orbits; thus, their velocity dispersion may not reflect the EFE at their instantaneous distance from the cluster centre. In this work, we simulated UDGs within the Coma cluster in MOND, using the Phantom of Ramses (por) code. We show that if UDGs are initially at equilibrium within the cluster, tides are not sufficient to increase their velocity dispersions to values as high as the observed ones. On the other hand, if they are on a first radial infall onto the cluster, they can keep high-velocity dispersions without being destroyed until their first pericentric passage. We conclude that in the context of MOND, and without alterations (e.g. a screening of the EFE in galaxy clusters or much higher baryonic masses than currently estimated), we find that UDGs must be out-of-equilibrium objects on their first infall onto the cluster.