Neptune-sized exoplanets appear to be located in three distinct regions when plotted in the period-radius space: the desert, which is an ultra-close-in region nearest to the host star and is typically defined by orbital periods shorter than 3.2 days and is characterised by a scarcity of planets; the savanna, in which planets reside at a safer distance from their hosts and are found in moderate abundance; and an intermediate region called the ridge, which features a surprising statistical overdensity. The formation and evolution pathways by which a planet ends up in one of these three regions are still poorly understood. This makes any new planet that is discovered in these regions, especially in the desert, a key new laboratory. We report the confirmation of the TESS transiting-planet candidate orbiting the metal-rich F8IV-V subgiant star TOI-5646 (V = 11.38 mag; Teff = 6136 ± 51 K). The planetary nature of TOI-5646 b was confirmed by means of HARPS-N follow-up radial velocity measurements. We determined that the planet has an orbital period of 2.427702 ± 0.000025 days and an orbital eccentricity compatible with zero. With a mass of 45.9 ± 4.4 M⊕, a radius of 6.6 ± 0.3 R⊕, and an equilibrium temperature of 1935 ± 29 K, it is one of the hottest super-Neptune exoplanets ever discovered in the Neptune desert. Its density, 0.87 ± 0.15 g cm-3, is lower than that of most other desert dwellers, but still appears to follow the trend that the desert is populated by dense planets than those in the ridge and savanna. The metallicity of the parent star ([Fe/H] = 0.32 ± 0.06) follows the trend whereby the host stars of Neptune-sized exoplanets in the desert and the ridge are generally richer in metal than those in the savanna. The log g of the parent star, 4.09 ± 0.08, suggests that the host is an evolved star, making this a peculiar system: TOI-5646 is the largest star (~1.8R⊙) ever discovered to host a Neptune-sized planet in the desert. Internal structure modelling revealed that TOI-5646 b is highly enriched in metals, with an inferred bulk metallicity between 0.7 and 0.95. Atmospheric evolution simulations indicate that the planet has undergone significant mass loss (6--16%) through intense XUV irradiation from its host star. These results indicate a peculiar formation history (potentially involving early protoplanetary disk dispersal), a planetary collision, or a late tidal destruction event, to explain its current state as a low-density survivor in the Neptune desert.