Any explanation for these forms? I was unable to find any in Smyth. Everywhere else (or almost?)the regular uia is used.
The Cambridge Grammar of CG notes that these are irregular forms 18.32
So rather less information than Smyth, unfortunately. I’ll see if I can find anything elsewhere.
ἑστῶσα and τεθνεῶσα are formed on the corresponding masculines (ἑστώς, τεθνεώς), which are much more common than the feminine. (So too with βεβῶσα and γεγῶσα.) So it’s a kind of assimilation, a readily intelligible phenomenon.
τεθνεῶσα appears to be the default form in classical Attic prose (LSJ cites Demosth. and Lys.) rather than the long-stem -κυῖα form which unsurprisingly is also attested (LSJ cites τεθνηκυῖα from Hipponax and Eurip., cf. Homeric τεθνηυῖα). The coexistence of both forms is the result of opposing pressures.