Last modified: 2014-02-12 23:52:47 UTC
The form should link back to the article you were viewing, or failing that, the home page - but both should be on the mobile domain, if a mobile-specific domain is available.
Actually, the thank you page itself appears to be on the non-mobile domain (at least when tested from en.m.wikipedia.org and de.m.wikipedia.org)
Fixed in https://gerrit.wikimedia.org/r/#change,6313
Taking Arthur's word for it that it is fixed :)