The post composer in Post Planner will keep any drafted content until you either create a post with it or delete it by clearing the composer.
This allows you to go back and forth between the composer and the rest of the app if you want to, for example, select multiple images one at a time in CONTENT, or go search for a text quote in CONTENT to add to an image, etc.
So if you want to delete everything in the composer and start over, you can't do that by just closing the composer.
You have to click the CLEAR button at the top right of the composer here:
Be careful! Once you clear the composer, the content is gone!