Why do some websites render emojis as symbols? (ex: warning emoji "⚠️")

Question

I'm experimenting an odd behavior for some emojis (the ⚠️ emoji, in particular, gets converted to its symbol version).

What I cannot explain is why it works on some websites and not in others.

For instance, using the MacOS "Emoji finder" (cmd + ctrl + space) and selecting the "warning" emoji, it displays the emoji in Asana, but it converts the emoji into it's symbol version in StackOverflow:

⚠️ (I would expect an emoji there, not a symbol)

I'm merely wondering why it works "as expected" in some websites and not in others. Does it have to do with fonts?

Here is the expected emoji

I'm experienced that behavior with Chrome and Brave browsers, under MacOS Monterey 12.2.1.

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Edit: Here is a video that hopefully explains things better

Edit 2 Here is what I see:

Answer 1

It is because how browsers renders the character, the reason why you see different representations of the same character with the same browser can be probably, some websites are appending "unicode variation selector": ︎ at the end of the character to be able to style it freely without relying on browser to render it.

A more detailed explanation: https://tonsky.me/blog/emoji/ and an example: https://mefody.dev/chunks/emoji-to-text/