Took me a minute to figure out what you meant. The instructions are shown on the post page or wherever.
Please report this bug here https://www.advancedcustomfields.com/contact/
Steps to reproduce
1) Create a field group with location of Post Type = Post
2) Create a text field with instruction and save field group
3) Duplicate the text field and save the field group
4) Change the copied text field to a message field and save the field group
5) Go to edit or add a post, instruction are shown for message field
