After ni the second member has the acute accent (udātta) on the last syllable when the sense is of 'not laying down'.,
The word निधानं = अप्रकाशता 'not making manifest'. Thus निमूल꣡म्, न्यक्ष꣡म्, नितृण꣡म् ॥ These are either Bahuvrihi or pradisamasa. In the case of their being Avyayibhava, they would have acute on the final by 6.1.223. Why do we say when meaning 'not laying down'? Observe निवाक् = निहितवाक् as निवाग्वृषलः; निदण्डः = निहितदण्डः ॥ The force of नि is that of निधान here.,