Karnatakabhashabhushana 6.25.2

6.25.2

tatpuruṣasamāsē vartamānasya pūrvapadasya sambandhinaḥ antyavarṇasya lōpaḥ prāyēṇa bhavati. yathōcitāgamaś ca vidhēyaḥ.

For a prior member occurring in a tatpuruṣa compound, elision of its final phoneme generally occurs; and an appropriate augment must be prescribed.