Plone.comments viewlet not showing in 5.2 on custom content-type

I have a custom content-types in Plone 5.2, discussion mode is globally enabled, a related context object has some comments. The problem is that the plone.comments viewlet is not rendered. The viewlet manager of the particular object shows the plone.comments viewlet including existing comments:

-> if self.template:
(Pdb) self
<Products.Five.viewlet.manager.<ViewletManager providing IBelowContent> object at 0x7fcb8dd3ca20>
(Pdb) pp self.__dict__
{'_ViewletManagerBase__updated': True,
 '__parent__': <Products.Five.browser.metaconfigure.SimpleViewClass from /home/ajung/sandboxes/onkopedia_buildout-plone5/src/onkopedia.policy/onkopedia/policy/browser/ object at 0x7fcb8dd30358>,
 'context': <Connector at /onkopedia/de/onkopedia/guidelines/kolonkarzinom>,
 'request': <WSGIRequest, URL=>,
 'viewlets': [<Products.Five.viewlet.metaconfigure.NextPreviousViewlet object at 0x7fcb8dd3cf28>,
              <Products.Five.viewlet.viewlet.SimpleViewletClass from /home/ajung/sandboxes/onkopedia_buildout-plone5/src/onkopedia.policy/onkopedia/policy/browser/viewlets/ object at 0x7fcb8dd3c240>]}

The viewlet manager for IContentBelow shows my custom css_js viewlet which is also place inside the IBelowContent viewlet manager but I don't see the plone.comments viewlet here.

Any idea?

Already spend a huge amount of time on this issue so I gave up and copied the comments viewlet code into my own policy package, re-added it to IBelowContent and here we go: comments are visible again...completely weird :man_shrugging: