The name Macsen Wledig has been applied to two distinct persons. The first individual appears in gen. -8 of HG 2 as Maxim gulecic, see Why Bartrum’s dating … . He also appears in HG 4 as Maxim guletic and in BGG 11 as Maxen Wledic. 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. -5 as in the follwing fragment from JC 13.
Note, Cyngar son of Protec is an error in this pedigree list. The latter name is simply Cyngar’s cognomen. The table is in accord with Constantine the Great’s birth in 272 or 273 and the birth of Helena, a wife of Constantius Chlorus, birth between 248 and 250.