/title doesnt display messages on screen with subtitle

Please provide the commands you are using.

I'm not entirely certain, but I think OP is referring to the following:

Running

/title @a subtitle {"text":"this is a subtitle"}

What is expected is that a subtitle shows up on screen, whereas in fact nothing shows up.

However, running

/title @a subtitle {"text":"this is a subtitle"}
/title @a title {"text":"this is a title"}

shows both.