The affix ṭhan comes in the sense of 'appointed there' after a word ending with agāra,
This debars ठक् । The difference is in accent and want of Vriddhi. Thus देवागारे नियुक्तः = देवगारिकः, कोष्ठागारिकः, भाण्डागारिकः ॥,
अगार-अन्ताट् ठन्
Adhyāya 4 · Pāda 4 · Rule 70