Sorry if this is intended functionality, but other Quick Add things like due dates don’t follow this same behavior, so I think it’s a bug.
For example, if I use Quick Add to add the task “@Jeff this is a test” it will get assigned to Jeff (me) and keep the full text of “@Jeff This is a test”.
However, if you add a task “this is a test tomorrow” then it will add a task of just “this is a test” and set the due date.
Example of both: https://i.imgur.com/DcoAJxW.png. Both were added using the Quick Add syntax.
This happens regardless of whether you put the user at the beginning, middle, or end of the task: https://i.imgur.com/9c5189j.png
I actually just thought of how this would behave with multiple users and found that it apparently is only affecting my user. I created one task starting with “@Jeff @Samantha” and one starting with “@Samantha @Jeff” and both times it only left “@Jeff”. Example: https://i.imgur.com/gfU4Ksg.png.