Yii-Jan Lin suggests that Paul considered Junia an apostle in the same way he was by referencing their joint imprisonment and that Junia was "in Christ before [he] was."
Yii-Jan Lin, "Junia: An Apostle before Paul." Journal of Biblical Literature 139, no. 1 (2020): 209
In greeting individuals in Rome, Paul seeks to strengthen his bond to that community by naming and indicating close relationships. Hence, Stachys is his “beloved” (16:9), and Rufus’s mother is like a mother to Paul as well (16:13). In greeting Andronicus and Junia, whom others might consider his equals, Paul takes care to acknowledge their faith while at the same time alluding to his own impris- onment and, in the last clause, his own apostleship—the apostleship he described in apocalyptic terms just before in the epistle, in chapter 15. All grammatical, mor- phological, and historical evidence—the meaning of ἐπίσημος + ἐν + dative, the occurrences of the feminine name ΙΟΥΝΙΑ in ancient Rome, the understanding of the church fathers—point to a prominent woman apostle named Junia. In the con- text of Paul’s emphatic and sometimes strident defense and his claims of unique apostleship and authority, we can confidently understand Junia as an apostle before Paul.