The improved performance of the long-legged alternative divertor configurations on MAST-U, combining strong baffling and total flux expansion, results in stronger power and momentum losses. These lead to lower electron temperatures and peak particle fluxes at the target compared to conventional divertor configurations. The evolution of the divertor electron density profile in the detached region, measured via coherence imaging spectroscopy, is characterized by a density peak building up near the target, downstream of the detachment front, with increasing upstream density. The peak then moves upstream from the target as the target temperature decreases (Te < 0.3 eV). This behaviour is in agreement with simplified modelling based on the competition of different ion recombination processes and neutral drag acting on the plasma flow. Comparisons against SOLPS-ITER simulations generally show good agreement in the magnitude of the electron density profiles, both along and across the separatrix, but with some discrepancies in the shape of the profiles in the private flux region and near the target.