I’ve reported something similar a while ago. I’ve tried several times since then, and it seems replays work rather fine (even though most of the time it’s a loss because the seed has changed), but doesn’t work at all when it’s a “hacked-in” submission with incredible score.
Most of these scores are early in the game (Kithgard Brawl is most hacked in, Backwoods brawl second) and contains a long list of “viewable” replays that are in fact just crap. Ex : URL to the replay of Rusokrd, score 20 at backwoods Brawl. Here is what this url brings me :
Later in the game, when I can find a respectable player submission, it works fine, like the ones of KevinHolland or Glacian or NishaNoire. Never found a bad replay so far, exept crazy warrior ones. (tested about 20, not a complete test).