I did the same on my sandbox yesterday and templates are shown in the list.
Could you please check if customizing mentioned in note 1339967 - Document Template pop up for Fax/Letter shows empty list is correct in your case? Did you check the same in standard IC_AGENT role?
The list is filled in CRM_OI_TEMPL_RT/Templates view controller (Class CL_CRM_OI_T_TEMPLATES_IMPL) in method DO_PREPARE_OUTPUT. First, it runs query CMBOLinkQuery. Then it takes the result and looks for CMBODocumentRefRel or CMBODocumentRel relationed entities (CMDOCUMENTTEMPL context node and method ON_NEW_FOCUS). This is the list entries.