"Allow auto-reduction" doesn't do that, unfortunately. It just splits tiles into smaller pieces if the specified tile size will result in underutilization of available cores/threads, I believe. E.g. with default 256x256 tiles, if you are rendering a 512x512 image and have 8 threads, you want 8 tiles, not 4.
Auto-subdividing tiles would certainly be nice but seems to be more complicated than one would like as far as implementation. It's definitely a known feature request and either that, or some other way to render those "stubborn tiles", will hopefully be implemented in the future.
- Oshyan