In the Outline of Phonology (OP2) from the 1950s, Tolkien mentioned a late Exilic Ñoldorin sound change of ts > ss:

but in Exilic 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.