When text is formatted in a chat message (i.e. the player is assigned to a scoreboard team) and narrator is enabled for chat messages, the narrator will announce formatting codes in the message.

For example, if my character is assigned to the gold team, and I am sending a message "test", the text is formatted as: "<§6PlayerName§r> test" internally and displayed as "<PlayerName> test". Narrator reads this message as "6 PlayerName R says test". The § characters are presumably sent to the TTS engine as well, but they are likely ignored because it cannot pronounce them.

This behavior seems buggy to me, as a narrator should not realistically announce text formatting markers. If it should announce formatting, it should instead use more descriptive language describing the meaning of the formatting marker, but I doubt this is the intended behavior.