BTW, are you an admin of any Wiki?

It's most likely that your browser/device does not support emojis. This is not uncommon and is not a bug or even an issue with your browser/device. The only thing you could do about it is use a different browser/device, but it doesn't really hinder performance, so you don't have to do that.

I use a computer where Emojis can be enabled. Just use the Win+Period keyboard shortcut.

Hrm... odd. If it's not your computer, then it's probably your browser. Have you tried using a different browser?

I have done some digging and have a theory as to what is happening. First, I would like to say that after doing this, I think Discussions definitely needs a way to search by date. I had to do so much scrolling and "load more" to find this. Here are the relevant things to look at:

1. replies in screenshot
2. proof the original contains an emoji
3. another reply that uses an emoji

So what is going on? Based on Special:RecentChanges, it looks like the wiki was converted to Discussions on September 1, 2020. Note the different dates on the replies above. The one with the replaced emoji is before the conversion while the one with the emoji is after. This implies that the issue is neither with the feature nor the viewer's device/browser but with Fandoms Forum-Discussions conversion script. This is very concerning to me because between the current and past versions of the script, Fandom has already converted a ton of wikis. That means, more likely than not, the majority of conversions have this issue.

I use the Microsoft Edge for Windows 10.