In The Etymologies from the 1930s, Tolkien said that for s + f in (late) Quenya compounds, the s was lost. This phonetic rule was to explain the name ᴹQ. Olofantur:

ÓLOS- “dream” ... Olofantur (s-f > f) = Lorien. ... [N] Olfannor (= Olo(s)-fantur) ... (Ety/OLOS).

The development of N. Olfannor seems to indicate this sound change also occurred in the Noldorin name’s history, but I think this more likely represents the usual loss of final s in ancient Noldorin/Sindarin; compare it with the isolated word N. ôl “dream” from *ólos. This means the loss of s before f was probably a Quenya-only change, and could not have been true of ancient compounds since aspirates became voiceless stops after s in Primitive Elvish.

Conceptual Development: This sound change was also mentioned in the Qenya Phonology of the 1910s: “Note sꝑ > sf > ff > f (PE12/21)”. It is impossible to say whether this rule remained valid in the Late Quenya of the 1950s and 60s, since the name Olofantur vanished form the Legendarium and the circumstances producing this sound change are incredibly rare.