Sindarin Phonetic Development (Part 55) [UPDATE]
NOTE: Edited to reflect timing issues, on whether this change took place before or after i-affection, and the implications on plural forms.
S. [x], [ɸ] vocalized between a vowel and [s], [θ]; [Vx{θs}|Vɸ{θs}] > [Vi{θs}|Vu{θs}]