Of a bha aṅga (stem) ending in an , the a is elided.,
As राज्ञः पश्य, राज्ञा, राज्ञे, तक्ष्णः पश्य, तक्ष्णा and तक्ष्णे ॥
Vart:- The elision takes place of a stem which actually ends in न्, therefore, it does not take place here, राजकीयम् ॥,
अल्-लोपो ऽनः
Adhyāya 6 · Pāda 4 · Rule 134