The problems happens on 2 AMS-hosted environments (sandbox and UAT). I haven't tried in our 6.5.6 prod environment yet as it was just given to us last week and I'm in the process of setting it up.
Only the built-in one shows up (in my case it's called Experience Fragment - screens variation). I know AMS has hidden some of the other built-in ones (example: Facebook, Pinterest and etc) but I do not know where they are hidden. But I do know the nodes exists (example: this /libs/cq/experience-fragments/components/xffacebook shows up CRXDE)
I've looked at the logs and nothing shows up. The logs only show that I've accessed the create page.
This experience fragment shows up fine in my local AEM running 6.5.6.
This experience fragment shows up in our AMS-hosted UAT/prod environments running 6.4.x.
You will need to add your template in the allowed template property. Please follow the below steps:
Please navigate to the required Experience Fragments folder.
Select the folder, and then Properties.
Specify the regular expression for retrieving the required templates in the Allowed Templates field. e.g. /conf/(.*)/settings/wcm/templates/experience-fragment(.*)? OR the experience fragment template path. (screenshot below)