If you run: /title @s subtitle "cool subtitle"
no subtitle will appear.
It will only appear if you run:
/title @s title "cool title"
So a subtitle only shows when a title is on screen. I included a video to show what I mean if it isn't clear. I attached a video showcasing this.
This is behavior has actually been in the game ever since titles where added but it looks like a bug. Because the command return text even says that it is "showing new subtitle for Playername" but if there is no title on screen the subtitle won't display. The message is lying.
It does not seem useful at all and only very annoying because now you have to run an empty title: /title @s title " " just to showcase a subtitle on its own. So that means an extra command just for showing another command.
And usually, you need to show subtitles on their own because the text doesn't fit into a title.