Please check if “Send notifications on button press or IR:” is enabled in Sync settings.
Unless this setting is checked pressing button will not update UI and WLED state with JSON API call.
This is by design.
Updated GH , i could not get that to work as expected on my test unit but might be the gentleman could test JSON APIs assigned to buttons , e.g {"ps":"1~4~"} · Issue #3204 · Aircoookie/WLED · GitHub