To reproduce, get any (?) Chinese character into a tokenizer token with a bunch of other tokens in the input. Here's one way.

• Change an account real name to "鴨".
• In a "Subscribers" tokenizer, subscribe this user and a bunch of other users.

This lays out like this in Firefox 73 (and sometimes Chrome/Safari if you zoom in or out):

The space under "duck" looks like it is caused by the "鴨" glyph making the token taller than the other tokens.