I answered this on AO3, so I’m gonna cut and paste:
Mafydd is technically pronounced with the ‘th’ at the end, like Lludd (which is a Welsh habit with the 'dd’), BUT, because upper class fae speak Queen’s English, they all actually prefer - in the higher classes - using the hard D sound instead.
So Mafydd prefers Mah-Fidd. And Lludd prefers that pronunciation too (Lud rhymes with hood).
But someone like Augus, who is also Welsh but was raised underfae, would pronounce the names properly everywhere but in the Court itself. He does pronounce Mafydd the way Gwyn does, but in his head he’s given it the 'correct’ pronunciation.
His mother who named Mafydd, preferred Mafydd to pronounce it the way it was intended. But Mafydd was a little shit. ;)