To expand on the previous response on behalf of Adobe, the Flattener Preview was never designed to convert text to outlines in the general case, but rather, as a side effect of flattening transparency in which text is involved. No transparency to flatten or no text interacting with such transparency, no converting of text to outlines.
However, there is a built-in Preflight function that will do conversion of text to outlines directly and very easily in one step.
Go to Print Production => Preflight. Select the group Acrobat Pro DC 2015 Profiles and under the section PDF fixups, you will find Convert fonts to outlines. Select that profile and press Analyze and fix. You will be prompted for a new file name (and location) and this profile will immediately convert all text to outlines for you.
- Dov
PS: Note that in general, conversion of text to outlining is absolutely not recommended as a general workflow practice. Text quality can be severely degraded. Text is not longer either searchable or editable within the PDF file. File size and RIP / display time can both dramatically increase. It is regrettable that some Luddite prepress professionals still believe that such text outlining buys anything at all.