Why does Proto-Indo-Aryan *ẓ seem to have different outcomes despite sharing the same phonological context?

Question

It is well known that Proto-Indo-Aryan *s had an allophone *z in voiced contexts. Due to some phonetic changes (i.e. RUKI law and the shift *śt > ṣṭ), they both could undergo retroflexion, thus producing two new allophones *ṣ, *ẓ. My question concerns the reflexes of the latter allophone in Sanskrit. It seems that the reflexes of *ẓ have a somewhat strange distribution.

Most grammars of Sanskrit agree that this *ẓ was eliminated from the phonetic inventory of the language in several ways depending on the contest: (1) In internal sandhi, it either was completely lost when it occurred before a consonant of the dental or retroflex series with compensatory lengthening of the preceding vowel, as in नीड nīḍá- < *niẓḍá- 'nest' (cf. OHG nest, Lith. lìzdas, OArm. նիստ nist, OCS гнѣздо gnězdo), or it was substituted by ḍ before other kinds of consonants, as in विप्रुड्भ्यः vipruḍ-bhyáḥ < *vipruẓ-bhyas dat./abl. pl. of विप्रुट् vipruṭ 'drop'. (2) In external sandhi, -iḥ, -uḥ (< *-iṣ, *-uṣ) are substituted with -r if they are followed by a vowel or a voiced consonant, e.g. अग्निरस्ति agnir asti < agniḥ + asti 'there is fire', or in the compound यजुर्वेद​ yajur-veda < yajuḥ + veda 'the Veda of the (sacrificial) formulas'. This sandhi rule can also be explained through a similar change where *ṣ > *ẓ / __[+ voiced], and at a later stage *ẓ > r.

If this account is correct, then in theory we should only find ḍ before inflectional suffixes beginning in voiced consonants, e.g. -bhiḥ instrumental, -bhyaḥ, -bhyām dative and ablative plural and dual, because as far as I know, when declining a noun, only internal sandhi applies. However, this doesn't seem to be the case. As a matter of fact, besides nouns like विप्रुष् vipruṣ- 'drop', द्विष् dviṣ- 'enemy', which indeed do have ḍ in their declension (let's call them Set A), there are also a few s-stems, e.g. हविस् havis- 'oblation', यजुस् yájus- 'sacrificial formula' and of the like, which have for example हविर्भिः havir-bhiḥ and यजुर्भिः yájur-bhiḥ (let's call them Set B), almost as if an external sandhi would apply here.

It seems to me that there is no relevant difference between nouns of Set A and B, at least from a phonological standpoint. More specifically, their phonological context is the same, i.e. an original PIE *s is retroflexed to *ṣ (because it is placed after i or u), which then turns into *ẓ whenever followed by a voiced consonant. Yet, they appear to show different reflexes of *ẓ, mainly ḍ for Set A and r for Set B. How is this possible? Can this apparent incongruence be phonologically motivated?

Answer 1

The differentiation is not derivable from synchronic phonetic context, it comes from the root vs. suffix distinction, where root-final s,ṣ dissimilate to t,ṭ, for example vas 'dwell' → (future) vat-syāmi; ghas 'eat' → (desiderative) ji-ghat-sa. We find examples like mās 'moon' → mādbhis. The divergent treatment of s in the ruki environment, ṣ → ṭ vs ẓ, is parallel. While dissimilation itself is phonetically explicable, a purely phonetic mechanism does not explain why suffixal and root sibilants are treated differently.

Kobayashi (2004, §42) has some discussion of dissimilation which tries to make the process be somewhat less irregular, but ultimately has to appeal to on-phonetic concepts of restructuring and the "obscuring" of roots (simplifying ss to s makes it hard to parse the root). A possible path for unifying the contexts s and stop after a sibilant is a presumed syllabification restriction that sbh etc is not a possible onset cluster: but again, that does not explain the root vs. suffix divergence.