Sometimes, when I’m writing the first characters of an emoji fast and I press tab to complete, the completion doesn’t work:
I’ve been encountering this issue many times and it also happens here on meta.
I have no memory of having this issue before the last weeks or months, so I guess this is some sort of bug (?) introduced in a “recent” update.
To reproduce, try writing for example :jo ou :joy and press tab, all of this the fastest you can. It can take several takes, but when writing posts “naturally” for me, the issue is frequent enough to be… Very slightly bothering 
(especially because it removes the focus from the text field!).