Recently I’ve been working with a client that was having trouble with their WordPress site removing paragraph tags from their posts and pages.
After some Googling, I found this: https://codex.wordpress.org/Function_Reference/wpautop
remove_filter( 'the_content', 'wpautop' );
remove_filter( 'the_excerpt', 'wpautop' );
By adding this to your functions.php file in your theme folder, it will stop WordPress from removing paragraph tags. Neat.