S. diphthongs [yi], [yu] became [ui]; [y{iu}] > [ui]
The diphthong yı̯ might arise in archaic plurals in some situations, either from i-intrusion or from the vocalization of ꜧ and ƕ, the latter producing some unusual plurals. I originally hypothesized this sound change back in 2019 as way of explaining plurals of monosyllables with long ô. These consistently have plurals with ui starting in the 1940s (as opposed to the 1930s when the plural was ŷ), for example: dôl → duil (SM/225), thôn → thuin (PE17/81), thôl → thuil (PE17/188). The relevant historical developments seem to be:
Additional evidence came to light with the publication of PE23 in 2024, in notes on the Common Eldarin Article (CEA) written by Tolkien in 1969 (PE23/133-140). These notes contained two more examples of ui plurals: dôr → i·nuir and gôn → i·ñguin (PE23/139), though in the first case Tolkien first wrote {i·nŷr >>} i·nuir indicating some uncertainty for this plural pattern (PE23/139 note #18).
In this document Tolkien also described vocalizations of sy > ꜧ > ı̯χ and sw > ƕ > u̯χ, which showed similar specialized diphthongal developments in plural forms. In particular, where singular osy- and usy(a) [> oı̯χ] > oech, the corresponding plurals had uich (PE23/139). Presumably the historical developments would be something like:
Somewhat curiously, Tolkien said that the plurals of words with -usye > -uich would instead have plurals with ȳch, but the historical developments here should have produced -usyi > -uich as well. Something else must have been going on, perhaps some kind of analogical leveling, or perhaps varying results in polysyllabic forms.
The sound change yı̯ > ui could have been limited to monosyllables, with polysyllables having unstressed yi simplifying to y instead, as evidenced by plurals like emyn [< *ambunī] and gelydh [< *ñgolodī]. There are a couple more examples, however, that seem to show ui diphthongs at the end of polysyllables resulting from i-intrusion that later developed into y:
Thus it may be that yı̯ > ui was a general rule, and then ui > y in the final syllable of polysyllables. Although there are plenty of Sindarin words with ui in final syllables, they are mostly obvious compounds which may have helped preserve the ui: Baranduin (baran + duin), Draugluin (draug + luin), Orodruin (orod + ruin), Thranduil (tharan + tuil).
As a final note, in CEA Tolkien also described the vocalization of ƕ > u̯χ, and where words resulting in uch also had the plural forms with uich (PE23/139). Presumably the developments were:
The simplest explanation is that yu̯ > ui just like yı̯, but it could also be that: -yu̯χ(i) > -yuı̯χ > -uich. Tolkien first had plurals with ȳch instead, indicating he was originally considering yu̯ > ȳ.