There is a time condition set up to choose between IVR (matches) and non-IVR (non matches) operation depending on time of day. The triggering time group period spans midnight (roughly 22:00 to 06:00 local time that varies based on day of week - see 20170101_fpbx_night_mode_settings.png for exact details).
A "temporary matched" override during times when the condition is normally not-matched will work when first activated. However, the override gets automatically deactivated within a minute of being overridden. Observation through the time conditions page shows that the deactivation occurs at the top of the next minute (0 seconds into every minute that passes). This occurs with both feature code and web site activation methods.
Testing Note / Workaround:
This bug is only reproducible with a time group that spans midnight. Inverting the logic in this scenario (i.e. creating a time group that spans 06:00 to 22:00 local time and inverting the routing logic) enables a properly functioning temporary override. See 20170101_fpbx_day_mode_settings.png for a functional time group example.
Time Conditions: 188.8.131.52