Multiple instances of opening / closing shortcode only works when closing tag is provided

[danielbachhuber]

Given the following example:

[url]Now this is wrapped[/url]

[url] This one is standalone

[url]Now this is wrapped too[/url]

The middle shortcode instance doesn't render unless you close it with [url/].

Attached is a patch with the unit tests. I'm unsure whether this is a fixable issue though.

[cfinke]

#50683 is a potential fix for this and other issues, if you'd like to review the patch there and give feedback.