When I search for a hidden achievement (i.e., one that hasn't had the prior achievement unlocked, in this case "Thaldraszus Advanced: Gold"), and alt-click or choose "Add to Watch List" from the contextual menu, it appears to be added ("Add" changes to "Remove"), but it remains invisible in the watch list.

I couldn't find an issue for this yet, and I'm not sure how doable this is (or if it's even worth it), but I figured I should file an issue anyway.

The reason why you don't see it is cause you have this filter checked.

What this filter does is only show the next one in the chain which for me would've been Silver.

The way I'll fix this one is add a new setting to have "apply filters to watched achievements".