I saw this today in two different instances. I'm not sure what is happening but there is a bug since the landing pages in question had been working just fine.
I was able to fix the landing pages with the problems by simply editing the landing page (use "Edit Draft") and then without making any changes, under landing page actions, selecting "Approve and Close". The urls on the style sheets fixed themselves and all was working fine on the landing pages.
Was going to send this in as a bug to Marketo support but didn't yet get a chance.