When a word's form conflicts with established Arabic morphological patterns, the defect called mukhalafat al-qiyas becomes relevant. Morphology (sarf) provides the relevant analytical tools.
The earlier example الأَجْلَل raised the question of the established rule of assimilation. But one must not condemn every apparently irregular form without examining attested usage (samaʿ), because some forms are established through transmitted Arabic usage.
Morphology identifies the rule; sound linguistic judgment also checks authentic usage and recognized exceptions.