Zhong is routinely translated loyalty, and the translation is not wrong so much as late. In the Analects the word's center of gravity is conscientiousness: doing one's utmost in whatever one has undertaken, toward friends as much as princes. Zengzi's daily self-examination asks whether, in planning for others, he has failed to be zhong, and the others in question are not rulers.
When a disciple asks about serving a prince, Confucius's answer makes the virtue bracing rather than servile: do not deceive him, and oppose him to his face. Zhong in the classical text is thus loyalty to the relationship's purpose, which can require resistance to the person; the loyal minister is precisely the one who remonstrates.
The narrowing came with empire. Han and later imperial ideology, ranking the bonds of society, promoted the ruler-subject relation toward absoluteness, and zhong migrated from conscientiousness-in-general toward political fidelity in particular, eventually hardening in some periods into the demand that a minister follow even a worthless ruler. The old meaning never vanished; the remonstrating official who accepts demotion or death rather than assent to wrong is a fixture of Chinese history and was celebrated as the truest exemplar of zhong. Modern scholarship works to keep the layers distinct, because reading the imperial sense back into Confucius makes the Analects an apology for autocracy, which its own text repeatedly refuses to be.