What masculine declension?
Everything I have read shows it's merely an accent that distinguishes between male or female and the jury seems out on what the accent should be except that...
sn The feminine name Junia, though common in Latin, is quite rare in Greek (apparently only three instances of it occur in Greek literature outside
Rom 16:7, according to the data in the TLG [D. Moo, Romans [NICNT], 922]).
The masculine Junias (as a contraction for Junianas), however, is rarer still:
Only one instance of the masculine name is known in extant Greek literature (Epiphanius mentions Junias in his Index discipulorum 125). Further, since there are apparently other husband-wife teams mentioned in this salutation (Prisca and Aquila [v. 3], Philologus and Julia [v. 15]), it might be natural to think of Junia as a feminine name. (This ought not be pressed too far, however, for in v. 12 all three individuals are women [though the first two are linked together], and in vv. 9–11 all the individuals are men.)
In Greek only a difference of accent distinguishes between Junias (male) and Junia (female). If it refers to a woman, it is possible (1) that she had the gift of apostleship (not the office), or (2) that she was not an apostle but along with Andronicus was esteemed by (or among) the apostles. As well, the term “prominent” probably means “well known,” suggesting that Andronicus and Junia(s) were well known to the apostles
Biblical Studies Press. (2006; 2006). The NET Bible First Edition Notes (Ro 16:6–7). Biblical Studies Press.
Metzger says
On the basis of the weight of manuscript evidence the Committee was unanimous in rejecting Ἰουλίαν (see also the next variant in ver. 15) in favor of Ἰουνιαν, but was divided as to how the latter should be accented. Some members, considering it unlikely that a woman would be among those styled “apostles,” understood the name to be masculine Ἰουνιᾶν (“Junias”), thought to be a shortened form of Junianus (see Bauer-Aland, Wörterbuch, pp. 770 f.).
Others, however, were impressed by the facts that (1) the female Latin name Junia occurs more than 250 times in Greek and Latin inscriptions found in Rome alone, whereas the male name Junias is unattested anywhere, and (2) when Greek manuscripts began to be accented, scribes wrote the feminine Ἰουνίαν (“Junia”). (For recent discussions, see R. R. Schulz in Expository Times, IIC (1986–87), pp. 108–110; J. A. Fitzmyer, Romans (Anchor Bible Commentary, 1993), pp. 737 f.; and R. S. Cervin in New Testament Studies, XL (1994), pp. 464–470.)
The “A” decision of the Committee must be understood as applicable only as to the spelling of the name Ἰουνιαν, not the accentuation.
Metzger, B. M., & United Bible Societies. (1994). A textual commentary on the Greek New Testament, second edition a companion volume to the United Bible Societies' Greek New Testament (4th rev. ed.) (475–476). London; New York: United Bible Societies.