Support has told me so far to try the usual tricks, clearing cache, updating iOS etc, but nothing works so far. The tech showed a video of them browsing the galleries but couldn't recreate the problem on their end which makes this harder to diagnose, but both my client and myself are seeing the same issue.
My plan is to change from a slideshow to a grid gallery, (better UX anyway) and hopefully it resolves itself that way.
Otherwise, I'm sure there's just some nebulous gremlins at play here that prob need special debug tools on iOS to truly figure what's going on here.