I have a 365 tenant where I need to have a workflow triggered on a custom date field set by the user, to do a review before the doc validity expires.
I've done the workflow in both 2010 and 2013 versions to test...
When I add a retention stage and go to 'Start a workflow', only the 2010 workflow is an option:
I've checked if it's the same for library and content-type based retention (it is) and when using a different date field (it is). Both of the workflows run OK if started manually (it just checks the fields are OK then sends an email).
I saw this on one of our 2013 O/P installs as well but never raised it with MS - it feels like retention wasn't properly interfaced with the 2013 onwards workflow engine.