I think this has to do with the fact that until very recently (last month?), while Amex Plat cardmembers could get into all of these worldwide lounges, and in spite of the fact that some of them may have been branded with "Centurion" branding, Amex only considered the US lounges + HK + London as the "core" Centurion Lounges. It was only a few weeks ago that Amex decided to more broadly market the entire worldwide network as "Centurion"-- at the same time they announced that the Escape lounges were going to be rebranded as Centurion Studio Partner.

Hopefully this means that access will be expanded for Reserve Card holders, but for the time being it sounds like Amex has some kinks to work through. I wouldn't be surprised if the non core Amex lounges have a laminated sheet or something showing which cards are granted access, and they simply don't have the Reserve Card on them right now.