WordPress appends the_content filtered text inside last paragraph tag of content (rather than after it)

Your filter is being applied before wpautop, the filter which wraps all of the paragraphs in WordPress content with <p> tags.

A value of zero for priority means that your filter will be applied first, before all other formatting filters. Try applying your filter later – with a priority higher than 10, the default, which is where wpautop is applied.

So, add your function like this:

add_filter( 'the_content', 'myFunction', 11 );