The [insert] mode in the Playground seems bugged for me currently. Upon submitting the prompt with the [insert] tag, the prompt text body is replaced with the result text, and the [insert] tag is moved to the end of the prompt.
The following response was received from Hello [insert] world.
Edit: Just a reminder that Insert mode is in Beta and that even if this were a bug, the correct place should be Feedback - OpenAI API Community Forum or the support.