The bug

/tellraw and /title fail when the message is longer than 1024 characters (including braces, whitespace and quotes).
This works in 1.12

Steps to reproduce

1. Try

   /tellraw @a ["",{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "}]
   

   This is exactly 1024 characters long. Note that it works.

2. Try

   /tellraw @a ["",{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" a "},{"text":" aa "}]
   

   This is 1025 characters long. It fails.