iOS 18: Why are some emojis sent as stickers and some not?

I think the new iOS update is terrible. It's not about the "new look," but rather how stupidly confusing so many things have become. Especially the gallery app… but back to the stickers.

On WhatsApp and other messaging apps, I often send hearts in different colors. Especially light blue. But even if I've written something, it's still sent as a sticker. It used to be that the emoji would only pop up in your sending bar if you hadn't written anything or drafted it, and then you clicked send. Now, when you want to add an emoji to the end of a sentence, it suddenly sends a sticker without warning. It's annoying and frustrating. How do you fix this?