Is the chat the old or new type of chat from Roblox, or Admin? If it is from Roblox and your testing you the new chat, there are reports of bugs occurring in them. Maybe use the old one for now? If it’s the old chat, I have no clue how to fix it, same goes with the custom admin chat.
Edit - I also hope this helps:The Big Bubble Chat Rework - #91 by Ok6c