In some cases in Ilkorin, an initial [s] before a consonant could become syllabic and develop into [es-], such as ᴹ✶spálass(e) > Ilk. espalass (Ety/PHAL) and ᴹ✶skal(a) > [eskal >] Ilk. esgal (Ety/SKAL¹). All of the attested examples of this development involve an initial [s] followed by a voiceless stop. In Tolkien’s notes associated with the Comparative Tables, he said (PE19/27):

Beleriandic partly shared this early tendency [of simplifying initial clusters] but in general shows its early connexion with Telerin in hesitation in this matter (producing both f and esp from initial sp)

In Tolkien’s notes on the development of the Beleriandic mode of Tengwar from the 1940s, he said (PE22/31):

in Doriathrin ... est etc. was the normal form taken by old PQ initial st

Both the quotes indicate that the normal development was [s] into [es] from an initial s + voiceless stop, though perhaps not universally. One possible counter-example is Ilk. istil “moon” which might have developed from *ṣtil < ᴹ√SIL. If so, the quality of the vowel that developed from the syllabic [ṣ] may have depended on the following vowel.