I have a Plone 5 profile in xmldirector.plonecore with old-style registries for CSS:
The CSS and JS files are properly registered with the plone-legacy bundle:

However I need to enable develop mode first (in portal registry setting, select the "Plone Legacy" bundle
and click on save (with "develop CSS" + "develop JS" enabled). After this manual step my CSS/JS becomes active. However this should happen automatically...anything missing here?
seems the same issue reported in #1511 and already solved in #1515.
Whats the state here?
@jensens I can confirm that this is true. The culprit appears to be this code from Products.CMFPlone.resources.browser.resource.ResourceView.ordered_bundles_result.
The list of bundles to process come from the get_cooked_bundles method of that same view. It includes the plone-legacy bundle, which has merge_with set to default. But when the process hits the test on line 202 in the highlighted section above, the test fails because production is set to True. As far as I can tell, this prevents any bundles include in any way (including the add_bundle_on_request function from Products.CMFPlone.resources) from being included. There appears to be no fall-back.
In production it appears that the only bundles that will ever be included are default and logged-in although it does appear that if the bundle is registered with merge_with set to None, or no value, that would allow the bundle to be included. I'll try that next.
I can confirm that if the registry is _not_ in development mode (production is True), and the legacy bundle is registered with merge_with=None, then the legacy bundle _is_ included in requests.
as mentioned, above; this has already being solved and we're just waiting for the release.