The “resume” feature only works for audio files stored in the Podcasts directory and in the Audiobooks directory.
(Does not work for audio files stored in the Music directory.)
The “resume” feature does not work using Folder mode.
The “resume” feature will only create one resume point for each audio file.
When you return to a resume position and continue playing, a new resume position will overwrite the prior saved resume position (for each audio file).
I have not tested the the “resume” feature beyond FirmWare 1.28.