User blog comment:Xilinoc/Ch. 586 - The Headless Star 5/@comment-4026406-20140625164406/@comment-1970092-20140627234446

It's said that the piece of Yhwach's soul that he distributes has engraved into it "the knowledge they had gained, the abilities they had learned through great effort, the skills they had awakened within themselves" (cnet128's translation). So let's say that Yhwach didn't give them their letter ability (though Shaz Domino and a few others mention that they Yhwach gave them their powers), he would still receive that power from them upon their death as the piece of his soul would have the power (among other things) engraved into it.

Now, I'd also like to point out that chapter 565 shows Yhwach giving Haschwalth his power, so I simply see no reason why Yhwach wouldn't get their power back upon their death.