Identify which resume action you used
Write down whether playback was paused within an open session or whether you returned later and selected a programme from its details page or Continue Watching. The interface may use similar everyday language, but those actions are different events for this setting.
Plex says Rewind on Resume can move playback a chosen number of seconds before the pause point when an item is unpaused. Its documented exclusion means that a later launch from Continue Watching can retain its ordinary resume behaviour without the setting having failed.
Check feature eligibility and the configured value
The publisher labels this setting as requiring Plex Pass and describes None as its default. Inspect the actual control and account feature availability on the HTPC application before expecting a chosen rewind interval.
For a controlled comparison, play an authorised item, pause it in the current session and then resume that same paused playback. Compare the result with a separate details-page launch, keeping the media item and selected interval constant. This establishes which action is affected rather than treating every way of returning to a title as identical.
Keep saved progress separate from the convenience rewind
The feature is a convenience around a pause event; it is not a general instruction to rewrite every saved viewing position. Do not clear watch history or change library metadata merely to test the documented difference between these two entry routes.
If the actual pause/unpause comparison still does not match the chosen setting, record the HTPC version, feature eligibility, selected interval and exact action sequence. For support, state explicitly that you resumed an open paused session. That description is more useful than saying Resume did nothing when the action came from an entry point that the publisher excludes.