It was a choice of implementation, in the reactions, despite being able to have any text, by default it is not expected to have anything other than the reaction in the content field so they judged the identification with : unnecessary.
Hum.. custom emojis are supposed to start with : maybe they missed this on their implementation 🤔
Discussion
Wrong call, though. The reactions NIP clearly requires the :: https://github.com/nostr-protocol/nips/blob/master/25.md
I think I was talking nonsense and they must have forgotten to add it. There have also been other emoji-related errors recently