The important thing to bear in mind is that WASP-76b is a gas giant (at the time of writing, the NASA Exoplanet Archive gives values of 0.92 Jupiter masses and 1.83 Jupiter radii, so this planet is much less dense than Jupiter is): there's no solid surface for the iron to build up on. Instead what would happen is that the iron would rain down until it reached depths at which the temperatures are sufficiently high for it to vaporise again. This would then allow it to be carried back to the dayside as iron vapour. Once it is back on the day side of the planet, the increased temperature would allow it to rise back up to higher altitudes, where the winds would then carry it back to the night side in an "iron cycle".
Superrotation is predicted to be a general feature of hot Jupiter atmospheres, though the detection of westward displacements of hotspots on planets like CoRoT-2b suggests that things may be more complex: see for example Debras et al. (2019) "Acceleration of superrotation in simulated hot Jupiter atmospheres" for some discussion of how this might occur.
There's also the question of the so-called "thermal tide", where the atmospheric heating affects the rotation. This effect is thought to be responsible for Venus avoiding tidal lock. On a gas giant there isn't a solid surface for the atmosphere to couple to, but in some cases this effect may contribute to at least the outer radiative zone of the planet being in non-synchronous rotation, see Auclair-Desrotour & Leconte (2018) "Semidiurnal thermal tides in asynchronously rotating hot Jupiters" for details.
