OS. [ns] became [ss]; [-ns-] > [-ss-]
Based on a single example, it seems that [ns] > [ss] in (Old?) Sindarin, since ✶rinsa > S. riss “cleft, cloven, separate” (PE17/87). In the history of Welsh, -ms-, -ns- > -ss- in Proto-Keltic (WGHC/§95iii), so Tolkien could have modeled the sound change after that. Most likely this is an early change, since it did not apply to the nasal-mutation for s:
- S. Echad i Sedryn “Camp of the Faithful” (UT/153).
[Hat tip to Elaran for pointing this example out to me.]
Conceptual Development: There are couple examples of ns in isolated names from Noldorin and Gnomish: N. Pensarn (TI/366) and G. Bansil (LT2/214), but the combination is rare enough that the exact phonetic developments are unclear.
Neo-Eldarin: It is probably best assume that this was an old sound-change and does not apply to later compounds.