In a custom mode, feature properties are gone when displayFeatures is executed

mapbox-gl-js version: 1.3.0
mapbox-gl-draw version: 1.1.2

Steps to Trigger Behavior

1. Create a custom mode. Override onclick method to add a point feature:

DrawSingleMeasurementLine.onClick = function(state, e) {
  // `this.newFeature` takes geojson and makes a DrawFeature
  var point = this.newFeature({
    type: 'Feature',
    properties: {
      meta: 'currentPosition',
      measurement: `100000000000000 km`
    },
    geometry: {
      type: 'Point',
      coordinates: [e.lngLat.lng, e.lngLat.lat]
    }
  });
  this.addFeature(point); // puts the point on the map
};

DrawSingleMeasurementLine.toDisplayFeatures = function(state, geojson, display) {
  console.log(geojson);
  display(geojson);
}

2. Observe that in toDisplayFeatures() the custom properties meta and measurement are gone, the console log looks like this:
   

Expected Behavior

Features added in a custom mode have all their properties persisted.
At this stage I can only fix this by setting the properties again in toDisplayFeatures, but this method is only executed when the mode is active. So, when the user deactivates the mode, the custom properties are gone and my feature doesn't render properly.
So it seems that addFeature() only saves the known properties.

Actual Behavior

Any custom feature properties are not persisting when adding to the map.

Start by reproducing the custom mode using newFeature, addFeature, and toDisplayFeatures as shown, and inspect the feature properties before and after displayFeatures runs and the mode is deactivated. Done means custom properties such as meta and measurement persist and the feature still renders correctly outside the active mode.