I was making a map and was using "/title" and then the game crashed. Crash report: https://pastebin.com/9HD7EQwU. I did test to see if the same thing happened in vanilla and it worked normally, but in forge, it crashed. To test for yourself, copy this command:
/title @p title {"text":"Test"}
It should display the words "Test" on screen, but on the most recent version on forge 1.16.1, it crashes it.