= Solution
A plausible explanation is \b[partial dissociation leaving a residual surface species], such as an NH$_x$ fragment, adsorbed nitrogen or hydrogen, after molecular ammonia has finished desorbing. A small residual population can maintain a surface <electric dipole moment> and a $0.2$ eV <work function> offset. Heating farther can remove, recombine or rearrange those species.
Test this hypothesis by recording <temperature-programmed desorption> signals for hydrogen and nitrogen as well as ammonia, and by measuring residual nitrogen with <X-ray photoelectron spectroscopy>. A surface-sensitive vibrational spectrum could distinguish NH$_x$ groups from atomic nitrogen. Isotopically labelled ammonia would help assign desorption products. Correlating disappearance of the residual spectral signal with recovery of the clean-surface <work function> would be more diagnostic than the absence of an ammonia signal alone. This is a proposed explanation, not a uniquely established identification.
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