Alright, so I did some experimenting and found a "solution," although it's not great.
I find that I can save a sprite just fine if I'm not logged into the browser version of Realm. That's why it was only happening in Chrome for me earlier, as well as why it had stopped working. As such, I and presumably others could create and save sprites, but it would have to be excluded from the list of sprites that we've made previously.
Obviously I'm waiting for a real-ier fix, but if you're desperate to assemble squares together, then that might help in the meantime. Let me know if this "solution" doesn't work, so that I can console you and say that I don't have any other ideas.