Q. exhilic [ts] became [ss]; [ts] > [ss]
In the Outline of Phonology [OP2] from the 1950s, Tolkien mentioned a late Exhilic Ñoldorin sound change of ts > ss:
but in Exhilic N. ts > ss (OP2: PE19/85).
This sound change was an addition in red ink (PE19/85 note #76), but it was also mentioned in the notes on the Feanorian Alphabet from the 1940s among “post classical” sound changes:
The colloquial development of ts > ss occasionally appears in use of k for 1Å (OP2: PE19/85).
This sound change was not mentioned in Outline of Phonetic Development [OP1] from the 1930s, where medial ts survives (OP1: PE19/50). Furthermore, the combination ts is quite common in Quenya, for example appearing in Q. otso “seven” (VT47/42; VT48/6; Ety/OT).
Neo-Quenya: For purposes of Neo-Quenya, I think it’s best to ignore this sound change.