Thank you for posting this question. I noticed the same <a> tag wrap anomaly yesterday; the system kept stripping the wrap. I'm curious if you get an answer.
This is actually a legacy left over from previous versions of HTML. It's only with HTML 5 that you're able to wrap a paragraph in an anchor like that, previously it would have been incorrect.
You can read the rules about tag order here: The global structure of an HTML document
The fascinating thing is that the English language has similar unspoken rules. For example, it's always "the big blue box." Saying "the blue big box" sounds weird.
You wont be able to wrap paragraph with HTML.
These are few weird rules of HTML. This happen with <TD>....</TD> as well.
If you want an alternative to hyperlink the whole strip on the LP; i would suggest you to replace the paragraph with the image.
You can not wrap anything except text or img tags. Why...Not because of Marketo or Modern HTML but because of that beautiful creature called outlook.
A few of the email clients only render/recognize the link if it is the direct parent element to the text or img. Making it a bad practice for email.
What are you trying to do create a full sized button?