The name Macsen Wledig has been applied to two distinct persons. The first individual appears in gen. -7 of HG 2b as Maxim gulecic, see Why Bartrum’s dating … . I believe this individual can be identified with the Roman emperor Constantius Chlorus as explained in the above link.
The Pen. 16 version of The Dream of Maxen Wledig only describes the story up to his stay in Britain. The version in Pen. 4 of the White Book of Rhydderch speaks about his return to Rome, but this continuation is actually referring to the second Macsen.
This is the Western Roman emperor Magnus Maximus born c. 335. He appears in gen. -4 as in the follwing fragment from JC 13.
|0||Cyngar (m.) Protec|
Cyndwr Fendigaid is St. Kentigern who was the son of Owain ap Cyngar and not Owain ab Urien Rheged. The table is in accord with the birth of Constantius Chlorus’s wife, Helena, between 248 and 250.