I can’t create any custom forms in lists in any sub sites in SharePoint Designer. It lets me create new display & edit forms in the top level of sites, but not in any sub site lists.
As a test I created a brand new list in a sub site of a site collection which just has the Title field. In the top level of the site collection (root web), I can create new custom forms for all the lists there. But in the sub sites I can't. Even in this new Test list that just has the title field. So there have been no customisations on this test list.