That's weird - yes, certainly there was signage at that roundabout and continuing east, but also further along: I took a couple of photos of the signage east of Garnlydan. https://imgur.com/a/czocdcI

I do remember it being missing on the path through the residential area at Rassau and on the entrance to the garage near the Brynmawr bridges/roundabout.

I don't think it's a leftover sign - quite the opposite: this is the (expensive!) new route built as part of the A465 works which hasn't been fully signed yet!