After looking into it for a bit, I've come to the conclusion that this has never been supported.
FieldTemplates are meant to be used to override, well, field templates, e.g. Twig blocks formatted as $FieldTypeIdentifier_field. simple_inline_field and simple_block_field are not field templates.
ez_render_field() will look for a field template in the configured ones, and *if it finds a field template override (<identifier>field block), use it to render the field. As a matter of fact, if a FieldTemplate contains both one or more field template blocks, overrides of simple_block_field and simple_inline_field _in the same template will be used.
But a template that only contains simple_block_field and/or simple_inline_field won't be picked up by ez_render_field().
I see that as a feature request, but not as a bug. And a useful one, gotta admit.
One workaround would be to define a FieldTemplate that overrides all the definitions, and just calls parent(), with a slightly higher priority than the one from EzPublishCoreBundle.