Some thoughts and notes so far.
This seems to be the only exception - which means it would most likely require an exceptional, ad hoc explanation.
Different sources trace back to the Greek grammarian Aelius Herodianus (2th century AD), the author of a most unusual compendium of exceptions Περὶ μονήρους λέξεως; see the following passage from A Companion to the Ancient Greek Language (ed. Bakker, 2010):
What other evidence do we have? As Goldstein 2013 writes, it's inscriptions and Latin transliteration practices.
Let's examine this kind of evidence in detail - I used Leslie Threatte's two-volume monograph mostly.
etc. [in progress]
Montanari's Dictionary of Ancient Greek (Brill) lists four words that start with the spiritus lenis:
He argues that
"initial smooth breathing by dissimilation, cf. Hdn. 1.546.21, 2.940.16 in mss. and edd. Ῥα- can also be found, prob. by analogy"
cf. the relevant passage from Allen 1987:
I also searched the Loeb and it seems it could be simply an editorial choice:
cf. ᾿Ρᾶρον (Pausanius) or ᾿Ρα[ριά]δος (Select Papyri) vs. τὸ Ῥάριον (Pausanius, mentioned in Montanari as "simpl. τὸ ᾽Ράριον" - note the difference) or Ῥάριον (Hermesianax).
cf. Dorandi 2006 "More complex is the situation concerning aspiration and accents which are both marked only rarely and then often arbitrarily. The spiritus lenis (smooth breathing) is marked less often than the spiritus asper (rough breathing)"
Lundquist 2013 reminds that
"These symbols [the spiritus asper and the spiritus lenis - Alex B.] were not in the autographs of authors writing prior to this convention, and texts so printed reflect Alexandrian editorial work. Concerning aspiration in the literary corpora, we should note that although our Ionic literary corpus is generally transmitted with aspiration on the model of Athenian texts (while Lesbian remains psilotic), this may in many cases be merely conventional."
He mentions an interesting example:
"all MSS of Semonides of Amorgos’ ‘On Women’ 82 give khōs, crasis of the particle ke and hōs, but a recent editor, Martin West ( West 1989), prints instead kōs to better represent Semonides’ East Ionic ōs.
Thus, he concludes by saying
"Whether we should print aspiration in Ionic authors remains in a number of cases disputed; as becomes clear from the evidence discussed above, our most reliable sources for psilosis are epigraphic."