From reading the comments, it seems to me that many of these "modern Welsh names" aren't used by the Welsh at all. They were probably designated by people who couldn't point Wales out on a map, just picked random words out of a Welsh-English dictionary so their kid would be the only "Rhamantus" in the class, not caring how ridiculous they would sound to a Welsh speaker. In some countries (namely America) people probably consider Welsh a "dead" language that nobody speaks anymore, thus feel free to annihilate it. I wouldn't be surprised if people started picking out random Irish words like "rómánsach" or "croi" ("romantic" or "heart"), and think those would sound like "unique" names for their baby.
It's a new name to me, and I am Welsh. And I just checked UK birth records, and there's not one single instance of this ever having been used as a name in Wales. [noted -ed]