Record the launched copy
Identify how the application was started and which executable the shortcut or file association selects. A portable copy and an installed copy can coexist. Keep that identity steady while testing, rather than alternating between windows whose configuration comes from different places.
An illustrative mismatch is editing the roaming file while a portable_config folder takes precedence for the launched copy. The file can be valid and successfully saved without controlling that session. Do not delete every configuration folder merely to discover which one matters.
Open and back up the active file
Use the documented Config and Open Config Folder route. Then identify whether the change belongs in mpv.conf, mpvnet.conf or input.conf. Preserve the original relevant file before making a small harmless adjustment. Keep scripts and private paths out of any shared example.
Repeat the same permitted local sample with the same launch route. Note whether the setting takes effect and whether the problem concerns the frontend, a key binding or playback. Avoid importing a large configuration from another machine to fix one unverified option.
Check remaining overrides
If the active file is correct, preserve the launch arguments and any relevant profile context for support. A command-line choice can be a separate part of the configuration story. Record what actually changes the result instead of assuming a successful save proves the setting was read.
Provide the app version, installed or portable type, active-folder route and smallest redacted configuration excerpt. Retain a working backup before further experiments. This article offers a location-first diagnosis based on the manual, not a validated configuration bundle or a test of every optional mpv.net integration.