After some testing, it appears that this error happens when the emoji_autocomplete_min_chars site setting is set to a value different than 0.
The error will occur each time we type a character after a colon. Also, the colon must be the first character of the line or have a space character before.
If emoji_autocomplete_min_chars is set to 3, the error will appear for the first 3 characters typed before the emoji selector is shown.
I donât know if it affects anything or causes unwanted behavior that I canât see behind the curtains.